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She fell sound asleep while reading her study guide.
Literal
She [topic-は] study reference-book [object-を] reading-while soundly fell-asleep [ended-up-てしまった].
Three pieces of grammar build the scene. ~ながら ('while doing') marks simultaneous actions, with the secondary action (reading) on the ながら side and the main action (falling asleep) as the predicate. ぐっすり is a state mimetic for deep, sound sleep — the kind where you can't be easily woken. 寝入る ('fall asleep') is itself a compound: 寝る ('sleep') + 入る ('go in / enter') — 'enter into sleep,' more vivid than the simpler 寝る. Finally ~てしまう adds a touch of 'unintentionally / regrettably' — she didn't mean to doze off. The whole picture is the universal student experience of nodding off mid-study.