Difficult decision as I have so many
Solar (theory or observational)
Rebel Sun by Colin Stuart, very readable and comprehensive overview of our understanding of how our sun works and what we can see on it.
Lunar (atlas, observational guide etc.)
Guide to the Moon by Patrick Moore, first book I got on astronomy
The Moon and how observe it by Peter Grego, what to see with various apertures and very interesting descriptions, one of the best guides I've come across
Lunar Atlas HP Wilkins published by Royal Observatory Greenwich, well this is certainly a challenge to use as it's so cluttered.
Planetary (whole solar system guide, individual planet history or spacecraft-related, observational guide etc)
The Planet Neptune by Patrick Moore, in depth assessment on the events leading up to and after the discovery of Neptune
Guide to the Planets by Patrick Moore, so many memories from the 30 years I knew him
Stellar (non-cosmology: star guide/atlas, individual star/nebula related, stellar evolution etc...)
Catalogue of the Universe by Paul Murdin, amazing pictures and bought just before I met Patrick for the first time, he kindly signed it even though he didn't write it
Jumbo star atlas, spiral bound and very useful when observing
Messier Objects by Stephen O'Meara, beautiful drawings and wonderful descriptions, his writing style is certainly engaging
The Caldwell objects by Martin Moberly, very useful guide with excellent descriptions and recommended equipment on how to observe and image them
Historical
The Astronomy of Burr Castle by Patrick Moore, absorbing account of Lord Rosse and his unusual telescope
On Stonehenge by Fred Hoyle, though provoking account of how Stonehenge could have been used as an observatory predicting eclipses etc., math's are at the back for those inclined.
University/College textbooks
The Moon our sister planet by Peter Cadogan, interesting analysis of the Apollo moon rocks by Peter who turned his doctorate thesis into a book.
ANY other books you just CANNOT be without
Sky At Night, I've managed after many years to get all of them, brings back memories when the program was watchable
1999 Solar eclipse signed by Patrick Moore, memories of a visit to Farthings
Remastered Hatfield Lunar atlas by Jeremey Cook, useful when browsing through the ALPO/BAA TLP schedule
21st Century Atlas of the Moon by Charles Wood, my goto atlas for things lunar along with Peter Grego's book