TOEIC Books Worth Buying
Three books are enough. Buying more usually means finishing none of them.
Official test collections come first
ETS-published tests are the only materials written by the people who write the exam. Everything else is an imitation.
One grammar book, one word list
A TOEIC-specific grammar book for Part 5, and a high-frequency vocabulary book you can carry around. That is the whole set.
How to actually use them
Work through the official tests under timed conditions, then use the grammar and vocabulary books to fix the gaps the tests reveal.
FAQ
Do I need the newest edition?
The current format edition matters; a one-year-old printing is fine.
Are free online materials enough?
They are excellent for daily reps, but at least one official test collection is worth the money.