I believe Stacy recommended: And strongly recommended (required?):
An Introduction to Modern Cosmology by Andrew Liddle
ISBN: 978-0470848357
Cosmological Physics by John Peacock
ISBN: 978-0521422703

Another text at roughly the level of Liddle's book is Barbara Ryden's Introduction to Cosmology (ISBN: 0-8053-8912-1). Also potentially useful, though out of print, is Eric Linder's First Principles of Cosmology (ISBN: 0-201-40395-1)

For serious cosmology grad students, consider beefing up your library with Scott Dodelson's Modern Cosmology, especially if you're into the CMB. Though out of date in some particulars, Jim Peebles' The Large-Scale Structure of the Universe (his fabulously detailed 1980 book; talk to me in person before buying Peebles' 1991 tome, "Principles of Physical Cosmology") is great for basic theory, and Ed Kolb and Michael Turner's The Early Universe (1990) is good for developing intuition and seeing definitions. Obviously, these books are pre-dark energy and even pre-no curvature, but they speculate enough in those areas.

For the diehard early universe (inflation and before), see Viatcheslav Mukhanov's Physical Foundations of Cosmology and Liddle and Lyth's Cosmological Inflation and Large Scale Structure.

You are welcome to borrow some of these from my bookshelf for brief stretches of time (I will still need them to write your lectures!), but for the gung-ho cosmologists, I recommend you go shopping.

Last Modified: April 2010 subject to change