Listmania!
Astronomy and Astrophysics
Universe 6th edUniverse 6th ed by William J. Kaufmann
Buy used from: £27.40
a good introduction to university astronomy, covers from the basics upwards, I used this alot in the first year of my degree
An Introduction to Modern Cosmology, 2nd EditionAn Introduction to Modern Cosmology, 2nd Edition by Andrew Liddle
Buy new: £14.78 / Used from: £11.99
I use this a lot at the moment and can find everything nice and easily
An Introduction to the Theory of Stellar Structure and EvolutionAn Introduction to the Theory of Stellar Structure and Evolution by Dina Prialnik
Buy used from: £25.00
a nice technical stellar evolution book, i find this quite approachable with just the right amount of maths
The Planet Observer's HandbookThe Planet Observer's Handbook by Fred W. Price
Buy new: £24.69 / Used from: £13.91
very detailed, if you want to look for planets then this is great!
Deep-Sky Companions: The Caldwell Objects (Deep-Sky Companions)Deep-Sky Companions: The Caldwell Objects (Deep-Sky Companions) by Stephen James O'Meara
Buy new: £26.99 / Used from: £15.49
if you are bored with Messier objects and want something a bit harder then try Patrick Moore's objects
Philip's Atlas of the UniversePhilip's Atlas of the Universe by Sir Patrick Moore
Buy used from: £4.37
Fun for all ages, not something I would use on a regular basis though
The Monthly Sky GuideThe Monthly Sky Guide by Ian Ridpath
Buy used from: £0.01
very useful! lots of detail and good star maps
Heaven and Earth: Unseen by the Naked Eye (Photography)Heaven and Earth: Unseen by the Naked Eye (Photography) by Katherine Roucoux
Buy used from: £15.75
a lovely book of pictures, has good descriptions of them