I started with Liu's grammer book and did all of the questions. Then I spent the holiday week to finish all of the OG questions. From my experience so far, Liu's book helped to divide questions by different areas. This at least tells me in which area I need most improvement. But I didn't go into his explanations too much - it confused me A LOT! I think OG is much better in helping me understand why I got some of the Q wrong...
(In hindsight I probably should have started with OG first and only skim through Liu's book. Oh well...)
do not worry. I was on the same boat, too. after finishing all the questions in Liu's book and OG, I improved a lot. same will happend to you. so just do it!