Reddit reviews Genus Cypripedium (Botanical Magazine Monograph)

For slipper orchids, one of the latest is Slipper Orchids of the Americas, a comprehensive and authoritative review of Phragmipedium, Mexipedium, and Selenepedium. It was released last year, so it's very up to date. It has sections on taxonomy, ecology, and evolution. It takes an in-depth look at each species, has lots of amazing full color photos and while it's very technical at times, it's also very readable and understandable.

The Genus Paphiopedilum is another book by Phillip Cribb, and as the title suggests, it's focused entirely on Paphiopedilums. It's a little dated now, and there are at least a dozen or more species that have been discovered since, but it's still a great resource. There are sections on ecology, evolution, hobby care, etc. Similar to above, there is a lot of technical terminology, but even if you aren't well-versed in the terms, it's still very readable.

Cribb has also produced Genus Cypripedium and Hardy Cypripedium: Species, Hybrids and Cultivation. The former is a monograph similar to The Genus Paphiopedilum and Slipper Orchids of the Americas and a very similar format. I read through the entire book several times back after it was first released, but I do not own it. The newer book was a surprise to me since I wasn't aware of its existence until just a few moments ago, but I can only assume it's a great source of information since Werner Frosch and Cribb are involved.

Tropical Slipper Orchids is a book by Harold Koopowitz (another important name in slipper orchid taxonomy and breeding). I don't own this one and have only seen it in passing, but it's perhaps a little more approachable for hobby growers and breeders. It covers Paphs and Phrags, and it differs from the Cribbs books in that it spends more time on breeding and hybridization (there's a short chapter in the Cribbs books, but nothing particularly exhaustive).

Anyway, many (maybe most) of the common genera of orchids have similar resources.