I've tried every diet written in the books, but my weight hasn't dropped at all.
Literal
Book [location-に] was-written-[state-ていた] all [attributive-の] diets [object-を] tried-[but-が], still at-all weight [subject-が] decrease-not-[ongoing-ている].
A complex sentence with several features. 書かれていた is the passive past continuous ('was written / had been written'). 全然~ない ('not at all') requires a negative. まだ~ない ('still not') is another common pairing. 減っていない ('hasn't decreased') uses ている to indicate the current (non-)state — an enduring failure rather than a single event. A vivid, relatable confession.