Next like others mentioned Alan Storm has put together quite a bit of information for you to soak up. He also has written both the book “No Frills Magento Layout” Edwin van Maastrigt mentioned as well as a multi-part Knowledge Base Getting Started Developers article, Which I believe is probably the best place to start amongst anywhere else if you are already familiar with ZF/PHP.
There are many Design Patterns that Magento uses and it definitely helps to have some in-depth knowledge into these. There is a good list of these here on StackOverflow.
You will also need a deep understanding of EAV (Entity Attribute Value) Data Model.
It also helps to have an understanding of how the end users work with Magento as well, familiarizing yourself with how the it operates both on the front end and back end will help you in the long run as well. Magento does offer a “Users Guide” for general purpose use.
Books (for me at least) tend to be on the “healing edge” and by that I mean they are usually about 3 months behind latest releases and may not always be on the “bleeding edge” of current releases and changes. Lots of books for 1.4, for example, and 1.5 is already out in the fray.
To answer your initial question: there are official Video Developer Trainings from Magento Inc, currently for free, that are worth a good look.
In a nutshell there is a lot of inside, that is Magento and don’t expect to pick it up over night unless; being already familiar with a lot of design patterns and data models would be the ideal candidates to get up to speed with using Magento. At times though it can still throw you for a loop. I have a massive amount of Bookmarks and still keep finding new things, with that in mind…
A few authoritative names to look for both on StackOverflow and else where would be: Alan Storm, Inchoo, Branko Ajzele, Colin Mollenhour, Ashley Schroder, Unirgy (Boris Gurvich), Clockworkgeek, Johnathan Day, Joseph Mastey, Fooman, and countless others on StackOverflow.