Thomas Harper Sr, along with his son, both wrote wonderful method books that are relevant to modern trumpeters. They explain the notation, transposition, ornaments, and technique in beautiful prose and outline a full program of study through their progressive exercises. This book was originally titled:

“Instructions for the Trumpet, With the use of the Chromatic Slide, Also the 7 Russian valve Trumpet, the Cornet à Piston or Small Stop Trumpet and the Keyed Bugle, In which Rudiments of Music and the Various Scales are clearly explained in a Series of Examples, Preludes, Lessons, Solos, Duets, etc. for each Instrument, Composed, Arranged, and Dedicated by Permission To The Right Honorable General Lord Hill, Commander in Chief, By Thomas Harper.”  – Original Title

Wow! That title pretty music explains everything that is in here (and you can tell they weren’t limited to 140 characters back then)!

Since this is not strictly a slide trumpet method, it is still very well suited to modern players. Great exercises, great duets and preludes, all around it is a wonderful addition to a modern trumpet method library.

Have a look at a whole bunch of samples to the left, and then grab a PDF download at the link above.