For me, this is an easy one: "The Night Sky Observer's Guide".

While at the Black Forest Star Party a few years ago I bought both
volumes direct from one of the authors for well under $100 (signed!).
As a relative newcomer it was at a time when I had just finished the
Messier list and was wondering "what next?"

This guide was just what I needed. When at home under heavily light
polluted skies I use the double star lists and am now a converted
double star fan - a real Struve head. When at dark sky sites I make
observing lists using the DSO descriptions and ratings to get a lot
more out of those rare opportunities.

Just what I needed.