That depends on your current level of knowledge. You need to start with a sound understanding of contemporary physics, including classical mechanics, electrodynamics, and quantum mechanics. It is unfortunately not possible to pick up a single book on grand unified theories without having gotten these other subjects under your belt. After these, you should study relativity (including some early universe cosmology), quantum field theory, and the Standard Model of particle physics. Gaining an understanding of the associated mathematics is also important; the big ones are: linear algebra, differential geometry, group theory, and eventually topology (Nakahara's book "Geometry, Topology, and Physics" is a good summary of the mathematics necessary to understand grand unified theories and string theory.)
If you seek a more popular-level introduction to modern physics, including cosmology, particle physics, and strings, the books by Hawking, Greene, Kaku, and Weinberg are mostly good.