Not using it much yet, I kind of need to get my C scale to actually come out fluidly first, and Klose seems to mix some more advanced stuff in pretty early. And my memory-building is a little slow at this stage, so a chromatic scale exercise on the2nd page of exercises is a little early. I would love to be practicing that, but the basics of the keys F, C, and G are keeping me pretty occupied for now. So I kind of have been using the watered down edition for now: https://www.amazon.com/Kloze-Lazarus-Method-Clarinet-Comprehensive-Methods/dp/1540001415/
Might come back later, it's just that a lot of more basic stuff isn't automatic yet. I suspect that the book I linked can be outgrown relatively quickly. The stuff in the back doesn't look insane, but it is a foreign instrument for now.
The easiest scales are probably F and G. C has you crossing the break, which I'm having to practice a lot.
I found this one was kind of basic but not childish, kind of assumes you know notation basics already, or at least doesn't waste too many words on it if not: https://www.amazon.com/Kloze-Lazarus-Method-Clarinet-Comprehensive-Methods/dp/1540001415/ It might be a little too easy if you can blow easily in all the registers but forgot some fingerings. It's basically notes and intervals and long tones, with occasional not-too-boring melodic snippets mixed in where they fit.