Some recent books on agaves (in addition to the one already mentioned):
Starr - Agaves (2012) great pictures, nuggets & wisdom. incomplete but deep coverage.
Moore - Aloes & Agaves (2016) more or less a picture book, showing the potential of these plants in gardens (mostly)
Spath & Moore - Agaves (2021) haven't seen it yet, also a picture book
Pilbeam - A Gallery of Agaves (2013) haven't seen it yet, but his other books have great pictures
The previous generations:
Irish & Irish: Agaves, Yuccas and Related Plants (2000) extremely readable, lots of useful details, good selection of photos
Gentry: Agaves of Continental North America (1982) the bible, now somewhat outdated, but still in print
