I am currently going through the Manhattan set of strategy guides. I created this resource, which is very helpful, for capturing section-wise OG-13 questions and answers given at the end of each strategy guide. These are essentially the OG-13 questions which are related to the concerned chapter in the guide.
I've found that these books can be very useful especially both for people learning the concepts from scratch as well as the ones brushing up their quants basics. Sure the concepts given in the quants guide might seem pretty rudimentary to guys with engg. or finance backgrounds, but there are some gems hidden in the mine of mineral.
The attachment is an OG-13 error log
inspired file for the strategy guide #1-3, with auto answer checking (answer grid courtesy GMAT prepnow) and optimism regarding reasoning behind each error
Hope this is useful. Do let me know if its useful I will put up similar files for the other strategy guides as well
When you feel like giving up, remember why you held on for so long in the first place.