、。
You lost the book — or rather, left it somewhere — are you sure?
Literal
Book [object-を] lost, [or-rather-というか] somewhere [at-に] put-came [nominalizer-という-の]-[topic-は] certain [is-です]-[question-か]?
Two linked constructions. というか ('or rather / or should I say') is a common discourse marker for self-correction or rephrasing. ~というのは is a compound nominalizer-plus-topic, framing the preceding clause as 'the matter of (having done X).' 置いてきた uses ~てくる in its 'action and return' sense ('put it there and came back without it'). A layered, slightly accusatory question.