I purchased ‘‘The Critical Reader’’, ‘‘SAT Grammar’’ (both written by Erica Meltzer) and ‘‘PWN the SAT Math Guide’’. I am planning on taking the test on the 6th of June, so I have approx. three months to prepare. What is the optimal way of studying for the tast? Should I study Reading, Writing and Math in parallel, or should I devote a certain amount of time to each part? Also, if I encounter words that I do not recognize while doing the sentence completion questions in ‘‘The Critical Reader’’, should I look these up or is it unlikely that they occur on the test?
I am extremely grateful for all answers 
Definitely look up every single unknown word you come across whether in your prep or in your outside reading. Make a list of these words and occasionally review them. The SAT tests extremely obscure words that are almost never used in any context, so you just have to take a shotgun approach toward vocab. Then again, don’t obsess over it because no matter how many words you memorize, the SAT will still probably have another bizarre and archaic word to throw at you. Vocab is good, but the sentence completion questions are a small fraction of CR.
A general approach to optimizing your study plan can only be made based on your individual strengths and weaknesses, but the SAT math is something you should definitely spend 30 minutes on each day. As you have a comfortable amount of time to prepare, you should go through math questions and compile a list of the questions you miss, then allow a good amount of time (maybe 2 weeks depending on your memory) to pass before retrying these same questions.
Make sure you do practice tests! That way, you can see which subjects you need to spend more individual time on.
Take your time reading those books. For vocab I will suggest you to get direct hits volume 1 and 2. You need to practice with the blue book. Learn each mistake that you made on the test.
Thank you all for the replies! 