I've never read the book, so I'm not sure if it will be covered, but this is normal behavior for auto_increment tables. The database will keep working just fine. Also, the is no way (at least, no simple way) to fill up those gaps. I wouldn't worry about it.
And no need to worry that you'll need those numbers because your table will get "full", because it goes up to 2,147,483,647 (that's for a signed int, for unsigned it even goes up to 4,294,967,295). If you're convinced that's not enough you can always go for unsigned bigint, which goes up to 18,446,744,073,709,551,615. But unless you're building the next twitter or facebook, you'll probably not going to be needing that