、。
While reading a book, I fell asleep.
Literal
Book [object-を] read-[while-ながら], I [topic-は] slept-[regret-completion-てしまった].
~ながら ('while') attaches to a verb's ます-stem and expresses simultaneous actions by the same subject. Distinct from ~ている間に (defined interval) and ~うちに (unspecified span during) — ~ながら specifically means two actions performed concurrently by a single agent. Here the reader was simultaneously reading and falling asleep.