So one thing I find really tedious as a DM is generating spellbooks and/or scrolls for my players to find. These are important elements for a wizard and I want to be sure to include them in my treasure generation, but again... tedious. Then it occurred to me that, with all the data the DDB team has access to regarding the spells most commonly found in a player's spellbook, they could generate a spreadsheet or something that would be a great resource for generating random spellbooks that are weighted towards the genuinely common spells. 

I know Adam often did data-dives like this in his videos, so maybe this has already been covered in one of them? If not, it might be a neat thing for the data team to provide for us at least in spreadsheet or tabular form.

This would be a fine dive for the Dev Update, if it's feasible.

I take a slightly more bespoke approach.  The spell book generally reflects who the caster was, and I don't really have "garden variety magic" users.  There are certain magic schools/clubs/societies where if you acquired a members spell book, there would be a common suite of spells, but beyond that I sort of have fun figuring out either a spell book that reflects a Wizards dedication to a type of magic or task or someone, likely an former adventurer whose contents are all over the place.  I treat spell books more like "lab books" so you can actually learn a lot about the Wizard's place in the world and their observations if the new owners spends enough time with it.  

Really, in my game unless the character is a mage slayer by trade, they're not going to come across a lot of spell books up for laying around.  They're not indestructible, so anything found in a dungeon is likely be fairly decayed damaged so only a small handful of spells could likely be salvaged out of it.  More likely their adventuring and politicking and sometimes just plain sycophancy is what gets them access to libraries of magic, and from there it's more what's not in it than what's in it needs to be determined for me.  But I do see your point and I myself would be curious what data could be rendered regarding spell selection.  

I just assume every country has spell libraries, with more prosperous cities containing spells of higher level, so any spell a wizard wants, a wizard can get.

I get that, and for more important 'named' antagonists I will take that same curated approach, but there are plenty of times where I just want to include a nameless wizard in my group of nameless warriors, archers, and skirmishers, but don't do so because generating their spellbook would take more time than their table-lifespan justifies and I don't want to use the same spell list in the generic NPC stat blocks every time either. Yes, there are spellbook generators out there and I have used them, but DDB has actual data based on the real decisions that real people have really made and that's fascinating. It's not something we've ever been able to do in D&D before, not really.

My ideal would be to use the data they have access to for breaking spells down by rarity, auto-filling most of the spellbook based on that rarity, and then manually adding a choice few spells that feel right to the kind of caster I'm making, even when it's a nameless goon. Something quick, like, "this guy likes illusions!" or "she dabbles in a little extra necromancy!" and move on to the rest of the prep.

Not sure if this is what you are after but.....

https://donjon.bin.sh/5e/random/#type=spellbook

Hit the generate button and quicker than you can say "I cat Fireball" a spell book is generated, you can even set the level and speciality/type of spells.

Quote from quindraco >>

I just assume every country has spell libraries, with more prosperous cities containing spells of higher level, so any spell a wizard wants, a wizard can get.

 i don't play this way or generally agree with it.  Level 1-3 spells? Sure, those are pretty common.  But lore-wise, most wizards are highly possessive of the magic they've learned and it takes years as an apprentice to learn the powerful stuff from a master (and then often by killing that master).  Just saying 'hey you want a level 7 spell, just go to the store' may work for some worlds, but I'd say its not that consistent with typical D&D.  Even in a place like Waterdeep, you can't just walk around and find high-level scrolls. Fireball? Sure. Wish? No.

imo.

So magical libraries are a sort of thing in my game, kept by magical orders.  They're not public libraries.  Interesting historical point, public libraries at least as we know them in the U.S. are actually a fairly recent phenomenon in the history of libraries.  Books were incredibly precious hand made endeavors and access was restricted.  Magical libraries are no different, access needs to be earned.  Sort of like in FR Candlekeep has the "hitherto unknown book" criteria for visits.  Other groups could require a quest for a rare spell component, delivery of.a new spell etc. It's very transactional.  But also allows for NPC/patron networking.

I would love to see the results of the data mining as well. What ancient libraries were like is harder to pin down. Yes in Northern Europe from 500 CE to Gutenberg libraries were rare and small - a “Great Library” might be 25-50 books or less. But at the same time in the Arab world there were real libraries at least partially open “ to the public” that contained hundreds to thousands of “books” of course even today the rare/special books are a lot harder to access than the common ones - I challenge you to try and get handle let alone “take out” the NYC library’s copy of the Gutenberg bible 😳😁. 

Honestly, a lot of the D&D Beyond pages (say, spells, monsters, magic items) could use a 'random' button, that just picked a random entry from whatever filter you currently have.

I think the OP is looking for less random generation and more "probability" generated.  I don't know if that's really great for spell books within an individual game world, but does work for the generation of "generic" spell books.

And yes, spell books would at minimum be accessible via the equivalent of "special collections" rules (i.e. like vouching from a library official or letter of introduction from a colleague, etc).  Some places will grant you direct access to the stacks or or however the collection is stored, but usually you're given a table, a library worker retrieves the book in question, it's delivery is logged and you're left with it for as long as protocol allows, sometimes under supervision.

The other option, for Arcane Orders tied to a state or other power, or just because their esoteric, would be to use the rules of security clearances and sensitive compartmentalized information.  Where what particular spells you have access for is based on the arcane order believing you have a need/right to know.