I am in the process of applying to various MCs myself. I found these two books quite useful for my McK test and would reccomend them if:
1. You're not from a numerical background (social science, etc...)
2. You want to polish up your basic maths (stats, percentages, etc)...
"How to pass graduate psychonometric tests" Mike Byron
"How to pass numerical reasoning tests"
I think they're quite commonly used by applicants, I found the latter more useful as it had some business case studies of sorts (which are closer to the actual tests, though don't take my word as for Deloitte), data interpretation, graph reading and so on...
Hope that helps!