Herring TCM + a couple of full articles each is week is all you need. The other criminal books (e.g. Simester and Sullivan) are far too detailed and engage in analysis that you will never be expected to do in an exam, especially if, like me, cirminal is a first year topic.
Having said that, Ashworth's principles of criminal law is good bed time reading just because its short, clear and he makes some good points, but on the whole it is less detailed than Herring's.