A book that disappointed you.
That's difficult, because there are quite a few of those around. Usually books which had a massive hype and fangirling going, and reading I discover they are absolutely not what those reviews painted them to be. I've since discovered that /usually/ (as in almost regularly) mass appreciation and mine do so not coincide and have begun to be leery of popular books with high rates.
This begins with FSoG and its endless imitators, glitter-BDSM books (in fact most books in the BDSM genre so far disappointed me), the several superstar novels and series among m/m, whether serials or not, books by those sons of famous authors who continue the work of their parents (Felix Francis and Todd McCaffrey I look at you!), a few major romance novels (Outlander, Vorgosigan Saga).
But I have to say one of the books which most disappointed me in recent time was "The Song Of Achilles".