The Beast Player - Nahoko Uehashi, Cathy Hirano

I don't like the English title because it removes Eren/Erin/Elin's name. Probably because it's to feeeeemale.

 

Overall the book relatively follows the anime, however the beginning of the book jumps straight to the action and the ending is vastly different from the anime. The book also doesn't have the two "commenter characters" either.

Overall the book gives a lot of perspective that the anime misses.

If you haven't watched the anime nor read this book, I'd say do both.