There are lot of alternatives
get.cab, order.cab, my.cab, go.cab, your.cab, the.cab, 1.cab, a.cab..
book.cab is nice domain name with good potential for obvious set of end users (any cab network can use it), and I would recommend to keep it in case domain renewal is < 100, if it is kept for potential resale purposes.
Of course, this advice is valid only when you do not have some development plans with it..in this case I guess renewal would not be that important (if one can afford it.)