。
Take the book and come down.
Literal
Book [object-を] hold-[sequential-て] descend-[gentle-imperative-なさい].
A sequential command linking two actions with the ~て form. 持って is the ~て form of 持つ ('hold, carry'), here meaning 'bring with you.' 降りる ('descend, come down') + なさい (gentle imperative). Common parent-to-child instruction when the child is upstairs.