http://www.amazon.com/dp/0470848359/?tag=pfamazon01-20 - This is a brilliant introduction. If you've never come across cosmology and are ~ 3rd year undergraduate then this is the perfect starting point in my opinion.
http://www.amazon.com/dp/0521422701/?tag=pfamazon01-20 - This is a very good book on cosmology. Perhaps a tad more dated than Liddle's book but covers all the important things in a very good way. Has introductory chapters on GR, QM and Field Theory that really serve as quick refreshers of some of the basics.