Jacobs' Low Register Studies for Tuba is the standard book on the low register of the tuba. This is the only book of it's kind and should be used by tubists at all levels of proficiency (except beginning). This book also contains material useful for developing the lower register on the high tubas (Eb and F).
"How to use this book: This book begins by presenting a group of exercises followed by a group of etudes. These exercises and etudes are repeated in lower keys later in the book. You are advised to master the exercises and etudes in their first presentation before moving on to the lower keys. Keep the air supported and moving at all times. Try to relax your entire body when performing this material. Listen closely to your intonation (as you progress to lower keys you will already be familiar with the melodies in the etudes). Use whatever fingerings work best on your instrument for the low register notes (don't be limited by the fingerings listed on the fingering chart). "
-- Wesley Jacobs
Catalogue No. T18.