I haven't really tried anything else so perhaps I'm not qualified to comment, but I've used Fitzpatrick 3U for Perms and Combs and found it explains the theory quite well using worked examples and the like. It's perhaps a bit short on questions, but you should be fine after completing what's there.
Mathsonline also has a Perms and Combs section with walk-throughs (which is good if you're struggling with basic theory) although it's in my opinion too brief to learn the whole topic from (which is brief in itself). It does have a few worksheets with worked solutions though so that will help pick up where Fitzpatrick lets down.
Either way I recommend getting Fitzpatrick 2U and 3U to go along with Cambridge, it seems to be a pretty good combination of textbooks for everything.
On a side note, does anyone know why Cambridge doesn't have anything on Permutations and Combinations?