The Wikipedia entry for NMR an excellent place to begin investigating NMR. NMR is a complex field and different people will have a different understanding of it. Something that one person feels is crucial to understanding it, another might find minimally relavent. Wikipedia is a collaborative project, so the explanations you find there will reflect the input of a number of authors.
James Keeler's NMR Lectures - Cambridge University, UK
The NMR Lab - Hebrew University
The Basics of NMR - from the Rochester Institute of Technology
Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Richard Ernst, Geoffrey Bodenhausen, and Alexander Wokaun, Oxford Science Publications. 1991
Understanding NMR Spectroscopy, James Keeler, Wiley. 2005
Spin Dynamics, Basics of Nuclear Magnetic Resonance, Malcolm H. Levitt, John Wiley and Sons, Ltd. 1991