the word 'book' here is functioning as a verb meaning to arrange (book: to arrange to have a seat, room, performer, etc. at a particular time in the future) the sentence would have the same meaning like this:
Uh, well, I haven't exactly arranged a gig yet.
In the context of the sentence 'gig here means performance (gig: a single performance by a musician or group of musicians, especially playing modern or pop music).