Under the guidance of my teacher (at the New England Conservatory), I am using a series of books called "Modern Method for the Violoncello" by Stephen De'ak. There are three volumes in total. I'm not sure what level they start at, but by the second book it has covered thumb position. The books include simple repetitive exercises for technique development, followed by a series of etudes which incorporate the specific technique; each book details several such techniques. There are also literary descriptions and instructions scattered wherever the editor/author saw fit, which I find to be accurate and reflective of my own teacher's teachings.
This website - http://www.cellobello.com/ - is also very helpful when it comes to proper technique in all the components of cello playing, and how it should affect the quality of playing. Once you get to the website, hover over the "cello lessons" menu, and select an area you'd like to work on. Most of these are 'taught' by Paul Katz, an illustrious cellist who was part of the Cleveland Quartet.
For anyone who's self-learning cello: it's easy enough to "play", but the guidance of a qualified teacher will get you a lot farther, as there are just so many bad habits you can fall into that will come into effect when your repertoire gets harder.