I'm currently a student game programmer working on an indie project. We have a team of eleven people (five programmers, four artists, and two audio designers) aboard, all working hard to help design this game. We've been meeting for months now and so far we have a pretty buffed out Game Design Document as well as much audio/visual concept art. Our programmers are itching to progress on our own end.

Each person in our programming team is well versed in C++, but is very familiar with C#. We have enough experience and skill that we're confident that we will be successful with our game, and we're looking to build our own game engine in XNA as it seems like it would be worth our time and effort in the end.

The game itself will be a 2D beat 'em up style game to be released over xbox live and the PC. It's play style will be similar to that of Castle Crashers or Scott Pilgrim vs The World. We want to design the game engine to allow us to better implement our assets into the game as well as to simplify the creation of design elements/mechanics.

Currently between our programmers, we have books such as "XNA 4.0" and "Game Coding Complete, Third Edition," but we'd still like more information on both XNA and (especially) building a game engine from scratch. What are any other good books, websites, or resources we could use to further map out and program our game engine?