One of the best books on Extreme Programming is Planning Extreme Programming by Kent Beck and Martin Fowler. It doesn't tell the customer how to gather requirements, but it gives lots of advice on how to do XP from the customer side... particularly Release Planning and Iteration Planning, and why, of four variables (Time, Scope, Resources, and Quality), he suggests only letting Scope vary.
Kent Beck's prose is highly factored -- he tends to say things once and only once -- so you have to read carefully. He doesn't explicitly define what a "release" is, but there are clues that he really does mean releasing to the end-user... when he talks about releasing to internal testing, or to the Customer (standing in for the end-user), he uses the phrase "interim, internal release". Here are few quotes from the book about how often to release:
Often the dates for a project come from outside the company:
* The date on the contract. * COMDEX * When the vc money runs out.
Even if the date is of the next release is internally generated, it will be set for business reasons. You want to release often to stay ahead of your competition, but if you release too often, you won't ever had enough new functionality to merit a press release, a new round of sales calls, or champagne for the programmers. (page 40)
Short Releases
Sometimes you can release much more often, maybe every iteration[....] However there is danger to never having "a release". The customer may lose strategic vision of where the software needs to go. (page 79)
Long Releases
What happens if you can only release once a year? [...] Another case is shrink-wrap software [...] look for a way to send intermediate releases to those customers that may be more interested in these versions. Call them service packs or something[....] Frequent releases are good, but if you can't release frequently you don't have to abandon XP completely. You may need to create interim releases that are only available internally. (page 80) [Bold emphasis is mine.]
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