"We're booking your holiday" v.s. "we're booking in your holiday"
Solution 1:
The verb book (in this sense) is transitive: it takes a direct object, the event (holiday etc) that is being booked. In is not needed, and may confuse your readers.
OED definition for "book" v:
(3b) transitive. To accept a reservation from (a passenger, guest, etc.) in advance of a journey, stay, etc.; to issue a ticket for travel to, or assign a reserved room, seat, etc., to; frequently in passive.