Once again, Hirsch nails it.
Clever, peace-loving persistent explorer Bartlett, his strong, stoic boyfriend Jacques, and their random sidekick Gozo, finish up their mapping of a massive canyon and decide as an afterthought to explore a giant tropical rainforest. While there, they find a fully sik city of....not gold, just like, food and peaceful living and stuff. It's nice! But is it... too nice?
In this book, there's more of a true villain, in the shape of the arrogant, obnoxious former rival explorer Elwood Tucker. There's also a vague not-love interest for Gozo.
Once again beautifully described (man, you can really feel that jungly jungleness! The vague Amazonianness of it all!) This book I'd say has a stronger message than the previous Bartlett books, more along the lines of Hazel Green's moral dilemmas, and one gets more worried about whether or not Bartlett and Jacques will make it out of the suspiciously nice forest city, but it all works out fine in the end.
In my memory, these two middle books are the strongest of the Bartlett series. I think I only read book 4 once or twice, maybe because I was ageing out of them at that point, maybe just because it wasn't as good. I'll have to see if it's got that same magic about it.
Also since very few people apparently have read these books or remember them, I can't talk to anyone about how Bartlett and Jacques Le Grand were DEFINITELY boyfriends. Outrageous!