The book has quite a few pages, and the book is red, and has Frown Digest written on the cover in dark purple.
Role in the episode
Squidward was seen reading this book near the beginning of the episode. SpongeBob then started talking about how he will earn Employee of the Month again but Squidward is seen not reading the book after SpongeBob was told by Mr. Krabs that Squidward could end up getting it.
It is a reference to the fact that Squidward rarely smiles.