Here are some books both about Protein NMR and NMR more generally which I have found useful over the years and would recommend for further reading. They cover much more of the theoretical and mathematical background to NMR than this website does.
by Malcolm H. Levitt
John Wiley and Sons (2008, 2nd Edition)
This is a classic text on the basics of Nuclear Magnetic Resonance. Malcolm Levitt writes in a very clear and approachable style.
by James Keeler
WileyBlackwell (2010, 2nd Edition)
James Keeler is known as an excellent teacher and lecturer, and this book forms a very good and accessible introduction to NMR. The lecture notes upon which the book is based are available on James Keeler’s website.
by P. J. Hore
Oxford University Press (1995)
As part of the Oxford Chemistry Primer series this is a very short text. It forms an excellent entry point to NMR and provides a very approachable introduction to the basic concepts of NMR including chemical shifts, couplings, relaxtion and chemical exchange. A new edition is now available.
by P. J. Hore, J. A. Jones, S. Wimperis
Oxford University Press (2000)
This is another Oxford Chemistry Primer, and again a very short text. It forms a nice complement to the first primer and covers some of the more mathematical aspects of NMR including Product Operators, Density Matrices, Fourier Transformation and Phase Cycling. A new edition is now available.
by J. Cavanagh, W. J. Fairbrother, A. G. Palmer III, N.J. Skelton, M. Rance
Academic Press (2006, 2nd Edition)
This is a classic text in the area of protein NMR. It is perhaps not as approachable as some of the books mentioned above, but covers both the theoretical aspects of protein NMR and the multidimensional experiments and pulse sequences available to protein NMR spectroscopists fairly comprehensively.
by Steven M. Pascal
IM Publications (2008)
A good book for solution protein NMR spectroscopists who want to understand the theory behind the many HSQC-based experiments they work with on a day-to-day basis. A particular strength of this book are the diagrams and animations by Jennie M. McKelvie which help visualise the magnetisation pathways.
by Melinda Duer
An excellent introduction to solid-state NMR for anyone starting work in this area.