The title is pretty self-explanatory, but I will elaborate a bit more just in case. 

So, this thread is about us sharing book series that either helped you get into the hobby of D&D (by giving you a vision of what it was like, etc) or otherwise inspire your playstyle in D&D. However, these book series have to be decently obscure or lesser known. No obvious series like Lord of the Rings, Dragonlance, Conan the Barbarian, Percy Jackson, Chronicles of Narnia, and other obvious examples. 

I'll start with three book series that fit my vision of D&D fairly well (or at least did when I started playing the game) in order to give some examples (if you happen to have read any of these series, let me know down below):

1. Adventurers Wanted - I have never met another person (other than my cousin, who introduced me to it) that has read this book series. It is very obviously inspired by Tolkien's works, the first book is practically a rip-off of the Hobbit (a group of adventurers go to track down a red, fire-breathing dragon whose name starts with "S" and ends in "G" in order to kill him in his mountainous, ancient city lair and take his gold), but the later books are more unique and compelling. However, the style of the adventures and the characters are much more important than the bland main plot of the first book. I highly recommend this series, there are 5 books, (the author wanted to do 7, but unfortunately he had a stroke while writing the 5th and thus had to quickly rap up the series in a less than satisfying way due to a stroke-caused difficulty to write), the first three of which are very good IMO, and the last two are at least worth reading if you enjoyed the first three books.  
   
   This series is aptly described by its name "Adventurers Wanted" contains an adventure shop where those with the skills of adventurers can learn about a diverse array of adventures and choose to sign an adventure's contract. How its adventures work is unique and possibly one of the best parts of the series. The series would be worth reading if only to learn more about its adventure contracts, how treasure gets divided amongst the party, and so on. 
2. The Books of Umber - This is another book series that I have never met anyone else that has read them. This trilogy of great books takes place in an in-depth fantasy world with awesome, unique magical creatures that are likely unlike anything else you've ever read/seen before, while also having some recognizable fantasy creatures sprinkled throughout (Sorceresses, Dragons, Giants, etc). I also highly recommend this book series, its overall plots are much more compelling than those of the Adventurers Wanted series and its characters are much more unique, compelling, and notable than those in the Adventurers Wanted series. If you like a bit of mystery mixed into your awesome fantasy adventure novels, this series is perfect for you, and is exactly what D&D feels like it should be to me; intrigue, horrible mysteries, fantastic creatures, and amazing characters with unique, yet relatable personalities. 
3. The Last Apprentice - This series is likely more well-known than the previous two (it got its own movie awhile back, although it bombed and has horrible ratings), but is nowhere near the popularity of the more obvious examples. This book series has a ton of books, 13 in the first series and 3 in the sequel series, which isn't including the 3 companion books. It has a much darker mood than the other two series, with an overarching theme of the series being a small force of Good against the hordes of Evil monsters and men, while also constantly forcing the characters to question where they draw the line between evil and good when innocent lives or the fate of the world might be at stake. 
   
   This series embodies the feeling of "everyone is out to get us" that many, if not all, D&D parties experience on their adventures, as well as the feeling of getting way over your head and having to deal with world-shaking problems when the odds are heavily weighed against you. 

Okay, those are the series that come to mind at the moment for me. I highly recommend all of them, and can't wait to hear what series are mentioned below. 

I think I watched the last apprentice movie, but I don't really remember it much.

The main thing that got me back into dnd after about a year of hiatus bookwise was probably rereading the ranger's apprentice series. I found the old book on my bookshelf and I forgot how good it was. I remember being the only person that actually read the series in my school so I think that counts as it not being mainstream. Anyways, it's really good and almost made me actually branch out an play a ranger in 5e.

Oh, wheel of time all the way. The world building is different(especially the magic system), but the story ideas are very similar

I have only read the first trilogy, but I've always wanted to run a game in the world of Brian Sanderson's Mistborn books. It would need some heavy homebrew, but I love the way the magic system works and the way you can combine the different powers in so many ways. Just a simple obstacle like getting over a wall would be a fun puzzle that could be solved in a bunch of creative ways.

You owe it to yourself to read the rest of Brandon Sanderson's books.  Honestly, the first Mistborn trilogy is some of his weaker writing, and it is still great.  The Stormlight Archive is the best fantasy of this generation.

I didn't really get into D&D because of fantasy books, though I read a lot of them. From the eighties I recall Glenn Cook (the Black Company), quite a lot of Mercedes Lackey (Valdemar), Diana Wynne Jones (Chrestomanci, Howl's Moving Castle, lots of others), Moorcock (Elric et al - way older than the eighties, but that's when I read them first), Raymond Feist (Magician and spinoffs), Tad Williams (Memory, Sorrow and Thorn), Steven Brust (Vlad Taltos), Donaldson (Thomas Covenant books - some sensitive subject matter there, not for everyone), Terry Brooks (Shannara), Robin McKinley (Damar) and of course Terry Pratchett (Discworld). Read a lot of D&D novels too, Dragonlance in particular.

None. I just wanted to learn how to play so I went to a gaming convention, played for 6 hours, and found myself a local DM 😄

On a side note, when I do read it's always non-fiction.

I have always loved fantasy books, but I will admit my very first direct experience with D&D proper was the Pool of Radiance game on the NES.

My 5 year old brain could not manage to figure it out, especially without the manual, but I'll be damned if I didn't try.

I'm old, so LotR.

Quote from BigDaddyMSU >>

I'm old, so LotR.

I don't think Lord of the Rings is "lesser known" ;)

I kicked off my adventures into Sword and Sorcery Fantasy 35 years ago with my (now late) Mum reading me these books!  

The Chronicles of Prydain

 https://youtu.be/rqqEu3oHapM

Still remember them like they where read to me yesterday. One of the greatest gifts my mother ever gave me! She then read to me the Hobbit... no turning back after then!

Conan (I was young!), Moorcock’s Corum, Hawkmoon and Elric, Fafhrd and Grey Mouser, the War of Powers (Robert Vardeman, warning NSFW!!!)

Well, not sure if they qualify as "lesser known" but the entire Forgotten Realms library is what I read.....and read again....and again, lol.  For me, however, I got into the books because of my love of the D&D world, from when I was a kid (The old Red Box)  I looked into several authors (Feist, Brooks, Chandler and others) before finding the Realms.  Never turned back

I haven’t found a bad book from Sanderson yet, and I’ve read pretty much everything but the Rythmatist and the Reckoners. His writing quality and his work ethic are so far beyond everyone else in the genre.

It was back in 1979 when i started reading it.

I'm going to add another series that I totally forgot the name of until today, Leven Thumps by Obert Skye. I also highly recommend this series, it has a ton of creatures that would fit right into the Feywild as well as having some of the strangest characters in a fantasy series that I have ever seen (trust me. Nothing's weirder than a magical, maniacally evil toothpick). 

Two obscure book/series that got me going:

Glory Road by Robert Heinlein (Available for FREE here: Glory Roadthe )

and four books in the Nick Pollota Bureau 13 Series. https://www.goodreads.com/series/58781-bureau-13

Both at least start out in the modern world (well, Modern when they were written) and start in with the fantasy pretty quickly.

Hey, I'm reviving this thread just to say that I started reading Brandon Sanderson's Cosmere books recently, and they're definitely going to impact how I DM my future games. I've even bought the Mistborn Adventure Game TTRPG and am preparing to run it with my players the next time we can get together. 

I'm not sure if I'd call Brandon Sanderson's books "lesser known", as he is a fairly popular author, but as it fits the main topic of this thread and I was recommended his books in this thread, I figured I might as well follow up on it. 

The Deryni books by Katherine Kurtz ended up being the actual bridge for me.  They were the first "fantasy" book series I read and opened up all of the other Fantasy Series and the game to me.  I still have a rather fond place reserved for them.