Based on my limited experience with EE I find that the open-ended nature and flexibility of the system means that the current documentation of the tags and their parameters should be sufficient to allow full development of a site in EE.
However, because of the open-ended nature I know that I’m missing out on using EE to it’s full potential, mainly because there is so much potential there. Therefore the type of book that I would love to see would concentrate on several examples of sites that have been built using EE and how they’re used EE in different ways. I was going to say “have examples of best practice” but I think there would be a lot of different best practices for whatever you’re trying to achieve with EE and promoting one above the other may actually close the door to a good avenue of functionality for some site owners.
Maybe the EE community can start posting “how I built my site and how I use EE” threads in the forums, just to give examples of how things are done. At the moment, I’m recommending EE to my ex-boss for the website that she’s now managing and I’ve been sending my templates to her PHP guy to show him how I use EE. I’ll expand this and start posting. As a quick example, I use static pages module for the main content pages, the weblog module for the homepage blog and the RSS module to pull in news feeds from other sites. My forums aren’t in EE but there’s no issues there (my site is http://www.oneofus.co.uk).