Book in Definition 1 of 3Pro Tip 1/3Pre-travel Action Used when securing a spot before arriving, like for hotels or events. Make sure to book in for the concert soon; tickets are selling fast! Pro Tip 2/3Advance Planning Reflects the need to plan ahead for accommodations or attendance. We booked in at the resort months ago to avoid the holiday rush. Pro Tip 3/3Confirmation Importance Highlights the certainty of having a reservation. After we booked in, they sent a confirmation email with all the details.