I only saw this now. Not sure of what you mean by easily readable, but María Elena Martinez has written quite a bit on the topic, and her article in Religion in New Spain might be a good place to start. Her book 'Genealogical Fictions' traces the concept of limpieza de sangre from which the casta system developed in more detail, from late medieval Spain to its implantation in New Spain, and its further development there up to the 18th c. There's also a recent book by Ann Twinam, 'Purchasing Whiteness', which looks more generally at 'pardos' and 'mulattos' in Spanish America.

For a more general introduction to the casta system you might look to an encyclopedia (e.g. The Oxford Encyclopedia of Mesoamerican Cultures), for the historical judicial documents to the Recopilación de Leyes de las Indias from 1680 which can be found online.