。
The books were bundled together.
Literal
Books [topic-は] in-one-bundle bound-[state-てあった].
ひとまとめ ('one bundle') can also be written 一まとめ — both spellings are valid. 束ねる ('to bundle, bind') is more specific than the general 縛る ('to tie'). 束ねる is more specific to 'bundling,' forming a tied-together group. ~てある marks the resulting state of a deliberate action.