Where do i get NAV 5.1 Prog: Function EBook?

Click on the help menu, then "C/SIDE Reference Guide", this contains most technical information about properties, C/AL syntax, etcetera. Then in the 'Doc' folder on the product disk, you'll find a document called w1w1adg.pdf, which is the application designer's guide, and that is our most important technical reference. Those two should get you a long way.

As for NAV 2009 - you can also use MSDN: http://msdn.microsoft.com/en-us/library/dd448639.aspx