I recently read Ian Grieve's book - Microsoft Dynamics GP 2013 Financial Management. This was an interesting read in that it covers GP functionality that is rarely covered in any materials that I've seen previously. When was the last time you saw any discussion about Analytical Accounting, Cash Flow Management, Purchase Order Commitments, or Encumbrance Management? Plus, all these features have been part of GP for a number of years, so the book's applicability is not limited to just GP 2013.
Ian talks about each of these topics, plus more, then describes the setup and use of the features in detail. One of my favorite features in GP is Cash Flow Management. I'm not sure why so few customers use it but it provides a very quick snapshot of cash inflows and outflows in a calendar format - quick and simple! The section on Encumbrance Management was interesting as well as I've never had occasion to implement or use that feature.
I highly recommend this book if you're looking for more information about any of these GP features. Pick up a copy at PackT or the following booksellers: