Incredibly inventive game!
It's a very cool idea, but it feel it might be a little too difficult right now. I think having 4 letters only to choose from is a little hard (could be a "Hard" mode, but I don't think it should be the default), so I believe the default would be 5 letter options, and maybe even an "Easy" mode with 6.
I think it could be cool as well to offer players the options to buy "powerups" at the end of a round. Some permanent would be buffs (1 free reroll per round, all letters are -$1 cheaper, First letter you drop has "[insert] ability] etc...), while others would be special rule breaks that are a one-shot (Replace a letter, erase a letter, place 2 letters instead of 1, etc...).
I only think this has potential to be a full release. Could do polish up the music, UI and animations, add a story mode about the wizard who made this book (mostly a narrative excuse to play multiple levels with different books, enemy types, letters, hek maybe even languages like a level where you have to make latin words), an endless mode and the option to play whatever music you want while playing the game, you have the makings of a cult hit. Also you could make have different varieties of backgrounds. Like a wizard coffee shop, or wizard lab or other things. And people put their hands on the table next to the book, and drop some coffee by accident, remove a pen, write a note on the edge of the page etc...
And you can tell I'm serious because I'm playing it again immediately after posting this comment!
(Edit: This comment was edited heavily because I initially had a request to allow players to select where they'd want to drop a word, I then realised after posting this comment that you *can* in fact, drag and drop letters anywhere you want. I had to remove that part. But in my defence, that wasn't explained in-game and I hadn't seen the .gifs on the main page (found this on the jam page instead)! So please explain that, as that changed the whole game! P.S: Maybe an add an 'undo' ?)