Quetglas has written at least three books:
Billar a Tres Bandas, 1993, 242 pp, paperback. The 1996 and 2001 reprints seem to be identical in content.
Estocadas Secretas del Billar, 2004, 270 pp, paperback.
Escuela de Billar, 1996, 199 pp, 1996. Reprinted in 2002.
The last seems to be fairly basic and includes sections on balkline and straight rail. The first two are mostly about 3-cushion, but the first has about 80 pages of introduction and discussion of diamond systems while the second has biographies and favorite shots of nine top players.
In looking them over briefly, I didn't notice any material that I hadn't seen in Byrne's books. If you are an engineer, you will be irritated by the diagrams.
I have duplicates of Escuela de Billar and Billar a Tres Bandas, but I would rather trade for books I don't have than sell them.
For an interesting 3-C book in Spanish, I'd recommend Andreas Efler's book, Billar a Tres Bandas Deporte Fascinante, which is available from several on-line sources.