(also btw I should have made it more clear in my original reply that I was focused on the learning-to-read part and not so much the learning to play walking lines part, which I have a totally different set of opinions about

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Reading anything when you're learning is good, I think, but Jim's books are really method books and not just reading practice. The method Jim used to teach sight reading-- well, at least, this is what I took away from him when I was studying with him, he never explicitly said to me "this is the method", and he definitely did not claim to have invented it, it's just what he used.
Anyway, the method that you learn in his reading books for electric bass is that he gives you an etude, lets say maybe its a piece in Bb major. Without any fingerings, you could play it wherever you want, and many new readers are accustomed to playing pieces with fingerings that would have you start down in the root position, since you're sort of working your way into knowing the fingerboard. Jim would have you make Bb on the E string your home position, and so if you have your second finger at Bb, in the sort of conventional major scale left hand shape, you now can cover everything from A on the E string, to Eb on the G string without any shifts.
For new readers, the area between the fifth and twelfth frets can be kinda scary, especially if you spend a lot of time down in the root position but are given a tune that is playing stuff above our high C and are having to shift lengthwise on the G. Jim would write etudes where it just made more sense to play in that home position, but he would keep working you in that area, not straying far from the key and focusing on intervals which fit neatly onto the staff in the beginning.
What would start to happen as you work your way through the etudes was that instead of having to look at the notehead and think "oh thats a A" and then to have to think about where its at in that unfamiliar area of the fingerboard, your eye would just sort of recognize that the vertical distance between say, the Bb and the A was a VII, and the right finger would just start to go there.
So really what he was trying to get us to do was to have a sort of formula for cold reading a piece; work out the key, work out the position, scan through it for places where we're going to have to shift or stretch for a note, etc. You could definitely learn to read without this method, but I think if you're teaching an assembly line of electric bassists who are lousy readers, you find out over the years how you can optimize the process. He was also quick to point out that double bass is an entirely different ball o wax; I can't remember if he was a Simandl guy or not because I didn't start double bass until many years after I had left Boston, but I think he took a different approach with it than with electric (maybe someone else who studied double bass with him can confirm)
But anyway, I like the books because they've got a lot of material that gives you practice in that, I guess you'd call it "position playing". It's obviously _great_ to read other stuff too, but Jim was good about stringing together etudes in a way that helped you with the crawl-walk-run progression, instead of throwing you into the deep end with solo transcriptions and stuff, although he definitely wrote a ton of those books too.
Anyhow hope that helps- it's a little puzzling because I don't think his books (at least not the ones I have) came out and explained the method he was using, although some of the other method books I've worked with over the years have been like that. I guess the idea is that if you play the pieces as written, nobody needs to explain anything to you, you just sorta learn it anyway.