Hola Connor

We sometimes say literally to book in somewhere using reservar en + place. It is as if we were omitting what we are booking, so, booking "a room" is implicit, i.e "reservar una habitación en un hotel" (reservar en un hotel) or "reservar una mesa en un restaurante" (reservar en un restaurante).

I hope this clarified it.

Saludos

Inma