Henri Couillaud's “30 Modern Etudes” or “Trente Etudes Modernes” is a fantastic collection of early 20th century trombone etudes by one of the most prolific composers of studies for the instrument. This book of etudes is preceded by a collection of scale studies by Paul Delisse.
The first 13 pages take you through a study or exercise in each major and minor key before moving on to 30 of the most engaging and throughtful modern trombone studies in existence. I highly recommend you check out the samples to the left, grab your trombone, and give them a try. You will be so hooked I think you will click above to grab an immediate PDF download (at a fraction of the price of retail as well). Enjoy!