Well, I can recommend a few that I found fascinating. I've always been interested in psychology and especially the subconscious (aka unconscious) mind. One book I really liked was
The Mind At Night by
Andrea Rock. That's a link to the Amazon page, though it's Amazon US - you might not be from around here judging by the name :chuckle:. It covers some fairly recent science of what happens in the mind when we're sleeping and dreaming. Actually there's a Nova documentary that covers much of the same material, it can give you a big preview:
https://www.youtube.com/watch?v=gj4JZ6lPZ74
Sorry, it seems to want to start well into the show - just scroll back to the beginning. I should mention, this book and documentary aren't about lucidity, just about sleep science, but I find that equally fascinating and of course they complement each other perfectly.
Another really interesting book that covers lucid dreaming and goes into different territory from the LaBerge book is
The Tibetan Yogas of Dream and Sleep, by
Tenzin Rangyal Rinpoche.
That's enough for now, but if I think of more I'll post them here.
On the same page with each of these books of course you'll see recommendations for similar books.