For me the following are some of the best:
Night Sky Observers Guide Vols 1-3
Herschel 400 Oberving Guide by Stephen O'Meara
Cosmic Challenge by Phillip Harrington
Deep Sky Wonders by Sue French
Local Group and Galactic Neighborhood Guide by the Astronomical League


The NSOG volumes are probably the best for looking up objects in. The O'Meara book is outstanding for doing the H400 and makes the program much more fun. Cosmic Challenge is a great book and will get you observing at your limits no matter your scope or binos. Deep Sky Wonders in a word is just gorgeous, and with Sue's writing pretty much a masterpiece. I pull that book out just to read all the time. Looks good on a coffee table too. Local Group and Galactic Neighborhood by the AL is the most professional guidebook from the League I have seen. The authors/editors really made it a labor of love and it shows.

So those are my favorites of the 100+astro books I have.