Vulgarization:

A Brief History of Time by Stephen Hawking (book)

The Inflationary Universe by Alan Guth (book)

Books (intermediate):

Gravity: an introduction to Einstein's General Relativity by J. Hartle

A first course in General Relativity by B. Schutz

Introducing Einstein's Relativity by R. d'Inverno

Books (advanced):

General Relativity by R. Wald

Gravitation and Cosmology by S. Weinberg

The Large Scale Structure of Space-time by S. Hawking and G. Ellis

Quantum Fields in Curved Space by N. Birrell nd P. Davies

On-line resources (advanced):

Lecture notes on General Relativity by Sean Carroll

Introduction to Cosmology by Mark Trodden and Sean Carroll

The Arxiv (the database of recent papers) by P. Ginsparg at Los Alamos / Cornell