。
The books were tied together tightly.
Literal
Books [topic-は] one-bundle [adverbial-に] firmly tied-[state-てあった].
一まとめに ('in one bundle, together') is an adverbial phrase. しっかり ('firmly, tightly') is a common adverb/mimetic. ~てある is the resultative/perfective auxiliary: attached to a transitive verb's ~て form, it marks a state resulting from a past action — 'has been done and remains so.' 縛ってある = 'is in the state of having been tied.' Distinct from ~ている on a stative or change-of-state verb.