I have gone through the powerscore CR
bible and gone through the OG questions.
The principles and types of critical reasoning explained in the book are just superb. If you have patience to read the whole 500+page book then it will do the job.
The explanations of answers are also very good- explains why a choice is wrong or right.
Relevance- The questions are much tougher than GMAT CR (at least when you compare to OG). So if you can do Powerscore the real GMAT CR will be breeze
Of course the best source is OG-10 and OG-11. The questions are representative of what you will get during the real test
Nobody dies a virgin, life screws us all.