Map projections are both easy and tricky. I am not a specialist at all about the subject, but I have used them a little bit.When I say that map projections are easy, I mean that, even without understanding them completely, there are tools which allow a user to get the work one. Of course, I use the Orfeo Toolbox library, which in turn uses the OSSIM library.
Actually, some years ago, with a student, we designed the interface in order to integrate OSSIM map projections (and sensor models too) into OTB so that they could be used as the existing geometric transforms already available in OTB (which came from ITK). The only thing we had to understand in order to design this interface was that map projections are coordinate transformations. Then, we chose the appropriate object-oriented design pattern together with a bit of C++ templates for the generic programming ingredient and that was it. When some users started using the projections (well, WE were those users) questions arised about the projections themselves. The task in answering to these questions would have been much easier if we had had this info (posted by Caitlin Dempsey on GIS Lounge):
The USGS has posted a scanned file of John P. Snyder's 1987 "Map Projections: A Working Manual" online in PDF and DjVu format. The beginning of the book contains substantial introductory information about map projection families and distortions. Each projection is started with a useful summary of the context and usage of that particular projection. Snyder then delves into detail about the history, features, and usage before providing the mathematical formulas used to calculate the projection.