The first three are quite different from one another--I started with #2 accidentally, right when it came out. It kind of depends upon whether NYC by night, deep-sea exploration, or baby computers scratch your particular readerly itch. :) #4 goes to Ireland (not great although Duane has lived there for decades), #5 adds cat wizards, and from there it's kind of remix/expansion of everything that came before, for better or worse. I still love #2 best (sea).