As you know the IMAT test deals with sciences (Biology and genetics, Physics & Math and Chemistry) and Thinking skills.
I would suggest a good Biology book such as the molecular biology of the cell which is a great text book for the IMAT and for first academic year
as a med student. In chemistry there are many great books, I personally like Chemistry and chemical reactivity, the subject are very clear.
For physics and Math I don't really have recommended books, however, some youtube lectures can clarify a lot, besides it's a small part of the IMAT exam.
some guys study also some physiology and anatomy and genetics as well. I can recommend some books in this fields, but in my opinion they are under "optional books". Last thing, for practicing the exam I will suggest books like BMAT 700 and thinking skills. Remember, practice is super important!
All of these books and other IMAT recommended books are available in online stores and are quite popular. Cheers.