I actually got a good basic ability playing in thumb position by working through George Vance's Progressive Repertoire Volume Two For The Double Bass. It is a book of well-known tunes, not endless exercises. Comes with a CD with Rabbath playing the tunes as well. By the time I was near the end of the book, I had a good understanding of thumb position and was able to apply it to jazz as well. This is not an exclusive thumb position book, but it was very valuable for getting comfortable in thumb position.