More Details Available on Google Classroom
Note: Etude ("etude" = "study" in French) Books can also be found on imslp.org for FREE, as they are public domain
Higgins Studies in Musical Expression, Vol. 1 (Supplement to Verroust)
Higgins Studies in Musical Expression, Vol. 2 (Supplement to Barrett)
Higgins Studies in Musical Expression, Vol. 3 (Supplement to Ferling)
Also, any of the Clarinet resources too!
See Also--Ferling 48 Studies for Oboe and corresponding Higgins (Above).
Also, under Trombone, Rochut Melodious Etudes (working on Bass Clef & Low Register)
If working on expanding lower register, look at Tuba resources too!