What is a booker and what do they do?

A booker is someone that answers the phone and setsup appointments for the ladies. They differ from agencies in that they are employees of the lady or ladies and do not take a cut of the business. Many independent ladies use bookers and most porn stars do so as well.

a booker takes the phone, answers emails, arrange appointments, make schedule, they get information and screen the guys before the lady goes to see them.
A booker might rent the hotel room for the lady, or drive her to an outcall location.

I would call a booker "personal assistant"

Is it profitable to get into with an agency? i.e. Is this somthing you can do to make money or is best used as a second job?

Not sure how profitable it is to be a booker. Some bookers do more than others. Most of them start out working for one or two ladies and expand by recommendation. They are usually the friend or acquaintance of their first client.

booker...in some cases refered to as a pimp.