The last really good manual (not reference manual mind you) was for TiVo. The Philips DSR6000 (same manual for a number of others, including DSR7000 and Hughes GXCEBOT, etc...) Really good read and informative. Not necessarily the best when you want very quick referennce on a specific function/command.

Its VERY difficult to keep up with documentation when you change software rather quickly. The best thing to do is to have a plan that includes the documentation and a long-term product lifecycle that doesn't see the product radically change such that the manual is obsoleted. Being able to leverage and add to an existing manual like the source code of the project itself is ideal. this can become very complicated with consumer-type goods with short lifecycles.

Bruno