Simandl - New Method Vol. I and II
Simandl - 30 Studies
Storch/Hrabe - 57 Studies
Sturm - 110 Studies
Charles Hoag - The New Book of Scales
Rabbath - New Method for the Double Bass Vols. I, II, and III
Simandl, Storch/Hrabe, and Sturm are pretty old while Hoag and Rabbath are more contemporary. My favorite is the Rabbath. These are all double bass methods but I have used them on electric bass as well. You will have to adjust for bowings. Many of the fingerings will stay the same. This should keep you occupied for the next 20 years or so.
