I found this book very helpful when I first got into Navision development. It's well written and gave me a good understanding of the main components. It is not a comprehensive guide, but a great introduction nonetheless! There are many programming examples and tons of screenshots. It helps fill in some of the gaps in the Application Designer's Guide.

I have it next to me whenever I'm doing develpment work. It's a good resource and I highly recommend this to anyone new to Navision.