I have two instruments - one with flats and one rounds. I alternate between the two every other day which generally means most (90+%) of all lessons get played with both. This methodology and my instructor (I also alternate the instrument I use at each lesson) keep me from getting very far off course.
On to book 2. 
just got my first Bass at 33yo. I love just playing around and by ear but gotta learn the basics and what im doing! Very excited to learn what's in here. Glad to see its been so helpful to so many on this TB thread.