The book <u>The Poker Tournament Formula</u> -- whether you buy the logic of the "formula" or not -- has the most comprehensive analysis I've seen of rebuys, add-ons, the theory behind them, and how to take maximum advantage of them.
An interesting aspect of a big add-on
not mentioned in that book (or anywhere else I've seen) is that it can substantially reduce the percentage chip advantage which those with higher stacks had before the add-on. For more on that,
see this.