A very steady seller in the Horn Notes Edition line of publications for many years now has been Playing the Wagner Tuba, a Handbook for Hornists. This is now available in an updated second edition as a Kindle ePublication. Purchase for $9.99 here.
A complete, practical guide to the literature and performance techniques of the Wagner tuba, geared toward the advanced student, amateur, or young professional who needs to get up to speed quickly to perform Wagner tuba in a symphonic setting. This edition includes information on instruments and fingerings, complete parts (original notation and transposed) for Bruckner’s Symphony No. 7, excerpts from Götterdämmerung and Eine Alpensymphonie, and more.
As the transposed Bruckner 7 parts are one of the selling points of this publication, I would also note that this publication was set up so that it can be opened outside of a Kindle as a PDF, using for example the program Calibre. From there it will be simple to print anything you wish.