Hey there. Not an easy situation to be in, but I'll try to answer your questions.

- No, it wasn't a "bad" thing to do, and there wasn't any real problem with it until your parent's (over)reacted, although their reaction is understandable.

- No, there are no detrimental effects of lucid dreaming that are known and proven yet, and it's not like it would hurt his development or his brain or whatever. There is ONE negative effect it could have, if he ends up being too obsessed with it and not living his life as a consequence, but that normally won't happen if he understands or you explain that lucid dreaming isn't a substitute for real life, just a way to have a good time or pursue certain goals or experiment.

-I haven't read the entire book, only excerpts I found on internet, but it seems perfectly suitable for someone that age. It's not like it's new age-ish crap, just gives you a better idea of the concept and of how to lucid dream. I don't think I would have shown it to my little brother if I had one, but that's purely because of personal reasons, I don't really mention LDing to people I know because I don't want negative situations to arise (Which doesn't mean they always will, I'm just staying on the safe side).

All in all, I don't think you did anything inherently bad, just things like lucid dreaming are relatively new and can appear like new age BS, although they aren't. The idea in itself seems kinda "weird" and so a lot of people like your parents might be (not without reason) concerned about it. I'd say the best way if you want to continue talking to your bro about this is discuss it with your parents separately in a calm way, even if they protest, and show that you understand how they feel about it, and explain it's not some weird, unproven/occult/new age thing and that you're not 'brainwashed'.