I'm looking to create an adventure in which the party is transported into a magical book. I haven't decided upon a theme as of yet, but I'd like to base it around some already existing creatures. Could anyone help provide some ideas as to what creatures may be found? Thank you! 

Well, the classic trope (from TV)  is to be transported into a book of fairy tales, indicating a bunch of fey creatures.

But I personally would make it a book about fossils and let you introduce all the dinosaurs.   Great way to explain why they exist in one location but not others.

As a bit of a cliche, I would probably go along the lines of the TV show "Once upon a time", basic premise is: Fairy tale characters are transported into the real world but have their memories altered via a curse to forget they are fairy tale characters. There is a hero of sorts that is destined to break the curse and return them all home and an "Author" who is writing the stories.

So for you're campaign you could have the party transported to a world that appears to be medieval with no magic, this is actually a pocket dimension located with a book. The party need  help different NPC's recall their true selves and break the curse to return home. Each NPC is a different fairy tale character and It would probably work with a Hag Coven or Archfey as the person to lay the curse and seal everyone away in a book and give a suitable antagonist to fight once the curse has been broken.

Depending on the level of Party the final villain could be a purple worm. This particular purple worm is tiny size but it has the power to bring the characters into the book as it slowly eats away the paper and remove the stories from the realms of men forever. An evil book worm.

Some ideas

• Fey are often based off story books and have a very fairy tale vibe 
• Dragons are a classic story monster 
• Humanoids , like knights ,wizards ect... are generic fantasy npcs that would be in stories too just really ham up the cartoonishness of them
• Talking animals as per the awaken  are a classic of fables 
• You can always pick an unusual genre like a gothis horror book and have a vampire or a werewolf. One of the domains of dread from van richten's guide would work for that
• One of the candle keep mystery quests Lore of Lorue has a similar premise of being sucked into a book. 
• Wizards made a supplement for wild beyond the witch light called domains of delight with a fey lord called Yarn spinner a giant spider who tells stories 
• The premise of tiny tina's wonder land is one of the characters tries to steal control of the story. Lots of meta jokes 

Instead of choosing one book, have the characters be sucked into an abandoned cursed LIBRARY.

This gives you, the DM, the opportunity to change up the story tropes as you see necessary. One adventure arc could be in with the Fey, the next it is dinosaurs, the next Spelljammer. Work out an expected mechanic for shifting settings though. It may be something as simple as "Every 4 long rests the setting can change" or "When this milestone is reached ..."

You could have the BBEG moving from storybook to storybook to complete their plan of implementing the Dewey Decimal system to force order on the disorganized "world".

Just a thought.

The party is forced to complete the different stories by the entity that entrapped them. So, first they have to get the talking sword from the wizard and use it to kill a fallen paladin, and then they have to save a city under siege, and then they have to set up some girl with her dream boy to complete the romance story (I’m making this up as I go along, but that makes it no less valid). My point is: have fun with it. This is a real opportunity to switch things around as you wish to.

I like the idea of either a library or a collection of stories.

One good idea is to make these stories reference real-life stories - change up the names perhaps but keep the plot - you'll find that the moment the players realise they're playing the Princess Bride, they will be extremely happy!

Plus you can pick old books that everyone knows but few poeple actually have read, like moby dick or dr jekyll and mr hyde, so the players start with "a white wyrm, and we're chasing it on a landship? This is moby dick!" and end with "how does moby dick end? I don't know how it ends!". You can also use stories with sad endings (titanic, jaws) where the party are trying to work out how to get to the next story before the bad bit happens!