It is a great skill to have. That Bernat book was a bit wonky - I worked out of it for years until British Airways lost a bag with it in it. I use the Fournier 'cello edition now. It is better, for me at least.
A good piece to get tenor clef under your fingers is this Fauré transcription - it is only one page, it sings really nice on the bass, has some nice challenges and is mostly in tenor after the opening in bass clef. That helps you get the main theme down before you take it up an octave.
(Just don't play it for a strict French teacher unless you really have it- I played it in a lesson with Joëlle Léandre. She was not happy.)
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