Your scores are fluctuating up and down. For example, on T3, your quant score stayed roughly the same as T2, but verbal dropped. On T6, by contrast, your verbal stayed roughly the same as T4, but quant dropped. (Note: I'm ignoring T5 because it was a re-take.)
So the issue is why your score sometimes drops in one section, so that we can (hopefully) prevent that from happening on the real test.
You'll need to dig into the data from the tests in order to try to figure out what happened.
First, read these two articles:
https://www.manhattangmat.com/blog/inde ... lly-tests/http://www.manhattangmat.com/blog/index ... -the-gmat/Then, use the below to analyze the relevant sections to try to understand the performance drops (V from T2 to T3 and Q from T4 to T6).
http://tinyurl.com/analyzeyourcatsCome back here to tell us what you think; we'll tell you whether we agree with your reading of the situation and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
Re: your request for books to study...we publish a bunch of books, so of course we think our own books are the best. :) You may also want to go to a book store to browse through some books or ask fellow students what books they thought were good.