Depends on your interests, too. I've drifted towards the non-tumor series since taking the boards, as that is often more relevant to my generally non-tumor practice. So I get more use out of the non-tumor respiratory book than probably any of the others, these days. Personally I found it hard to decide what to and not to get, so I subscribed to get them all as they come out. But I can't say I use many of the tumor books, nor some of the non-tumor books, on a regular enough basis to know them well. From those I've worked with who regularly do, the CNS tumor book has come up. YMMV.