。
I still don't have enough material to write a book.
Literal
Book [subject-が] can-write [extent-だけ] [attributive-の] materials [topic-は] still gather-not-[state-ている].
A tricky nested clause. 書けるだけの資料 = 'enough materials to be able to write [a book]' — here だけ is an extent/scope marker ('as much as / enough to'), not the 'only' sense. まだ~ていない ('still hasn't come about'); 集まっていない uses ている for state negation rather than simple past. The sentence expresses an ongoing lack of sufficient resources.