GMAT Preparation either involves extensive study materials or very few depending on your target score. If you are opting for self prep, then you should have these materials.
Must have materials
OG 12/OG 13. You can always supplement it with OG quant/verbal review and GMATPrep pack 1. You can get these here. There is no GMAT Preparation without OG.
Quant Study materials:
Foundations of GMAT Math by MGMAT/Kaplan GMAT Math foundations - one among these if you are too weak in quant say scoring in the 30s have trouble with basics
MGMAT - strategy guides/Veritas/Kaplan - one among these to improve from mid 30s to mid 40s. MGMAT tops the list.
Verbal Study materials:
CR - Powerscore CR - if you are having trouble identifying the question types, otherwise go for MGMAT CR. Don't get bogged too much with powerscore strategies.You will start segregating answer choices based on rules rather than on reasons, say for ex. you face a CR answer choice to any question you think about the probable answer choice types, then read the choice and instead of analyzing directly with the question you will try to fit the choice into one of the categories such as shell game answer, opposite answer etc. This is detrimental to your CR performance. CR problems should be solved intuitively and not in a rule based manner. Stick to powerscore for starters and then you can switch to MGMAT CR.
SC - MGMAT SC is a must, you can supplement it with Aristotle SC or GMATClub's Ultimate Grammar book, both are quite good but none can beat MGMAT SC.
RC - RC comes with practice, you have to devise your own strategy, some might find it comfortable to take notes, some don't even take notes, many people skim through the passages. Veritas RC guidebook is quite good, but as far as I have seen Ron's videos are the best.
If you are scoring in the mid 20s and need to improve your score to 35+, then its probably the right time to check out Thursday's with Ron videos - the best free videos one can get for non native english speakers.