I've found Mark Rosenfelder's books "the language construction kit" and "the advanced LCK" fascinating to see how much thought needs to go into a language. I fe

I've found Mark Rosenfelder's books "the language construction kit" and "the advanced LCK" fascinating to see how much thought needs to go into a language. I felt I learnt more about existing languages from those books than actual language books. He deals with everything - from syntax to phonology to script. LoTR was the book that drew me to conlangs in the first place though. The community is also fascinating, with folks spending passionate decades on making languages - ex: ithkuil.