I'd recommend, at the very least, having one good Sentence Correction book and one good Critical Reasoning book.
As for which test prep company you pick, the most important thing is fit. Take a look at the free material they offer, and if you find it helpful, consider purchasing the corresponding book. If not, keep shopping until you find something that suits your learning style.
I work for Veritas, so here's a link to our free Sentence Correction lesson:
https://www.veritasprep.com/gmat/free-gmat-lesson/
Then spend a lot of time with the questions in the Official Guide. Even with the questions you answer correctly, spend some time breaking down why the wrong answer choices are wrong. Pay attention to the traps the writers set. The same patterns will show up again and again. Anything that isn't clear, post in one of the forums here, and multiple experts will break the question down for you.