Which is the best ca final costing book for self- study?

hey, i hv done relevant costing n make or buy 4m his videos ( of 2009 or 10), i found the concepts very easy, bt when it came 2 solving questions 4m his book, i could solve hardly 40% - 50% on my own.. now i need some suggestions on what 2 do if i go 4 self-study..