。
She was reading a guide to gardening.
Literal
She [topic-は] gardening [genitive-の] handbook [object-を] was-reading.
手引書 ('handbook / guidebook,' literally 'hand-pull book') is close in meaning to 入門書 but with a slightly different flavor — 手引 emphasizes practical guidance, the kind of how-to manual you consult while doing something, whereas 入門書 frames itself as an introduction to a field. The verb 手引する ('to guide / show the way') captures the same image: someone leading you by the hand through a process. Past progressive ~ていた reports the ongoing state at some past time.