Packt Publishing have released a book on the Funambol Mobile Open Source system. I’m not going to write a review of the title, because I worked on it, so I can’t be completely impartial.
However, I will say this – I really enjoyed working on it. I use Citadel for my email, and it can easily integrate with a Funambol server. If you’re looking for a PIM and push email system that works with a variety of mobile clients, then you should definitely check out Funambol – it’s free and open source, and can scale to suit anything from a small business like mine to a much bigger enterprise.
The book takes you through the basics of setting up a server for personal use and scaling it for more users, then goes on to explain how to create Funambol extensions. You can check out the book here.