The question suggests that in the Reformed church we don’t use prayer-books or collections, but this does not fully match with our practice. In the Reformed tradition the use of written prayers was never rejected. It is true, that we have ‘free’ prayers in our services, but we use different collections as well. For instance we have a prayer of confession and we have collections in our forms for Baptism and the Lord’s Supper.

If you want to read more see:

Карел Дедденс, "Литургические молитвы в истории реформатской церкви"