Considering your background, this is a great choice as a way to get into game development.
As far as advice and suggestions go, here are some ideas.
What should I read?
There are quite a few sites out there that post tutorials and news regarding Windows Phone / XNA development. Most important (in my opinion) to get you started is the official Education Catalog at the AppHub website. These projects are updated with the latest SDK and have some great examples with full source to get you started. Others sites include:
- Sgt Conker (Not updated as much as it use to but still some great links and tutorials)
- XNA Development (Good tutorials, XNA 3, most lessons still apply even to Windows Phone)
Very broad question, try going through the AppHub Education catalog first for examples, if none agree with what you are trying to acheive, look here for old questions or ask a new one. Whatever works for you. Getting stuck in design (see Analysis Paralysis) is a horrible problem. Prototype, test, if it works for now, move forward, if not prototype again or move onto another problem and come back to it later. See this question/answer for good practices and some really good advice.
Third party libraries
Although finding a library is a good way of getting you up and running quickly instead of re-inventing the wheel, as a beginner, it's always good to start with something simple you can make yourself. Maybe base it off an example and work from there. If you find you want to be able to load a custom file type or overcome a specific problem that you think some one might have dealt with before, have search and if you find a library that solves your problem, use it. I wouldn't start with any libraries from the beginning unless you have a specific design problem in mind. Besides, you'll learn a lot (and hopefully have fun) by making everything from scratch :).
Silverlight(Only) vs XNA
XNA is a great framework for games and is supported by Windows Phone. I highly recommend having a crack at using XNA for your game. Windows Phone Mango supports using Silverlight and XNA together, so you have the best of both worlds. Again, have a look at the AppHub education catalog for examples.
Whatever I should pay attention to
Your game itself. If you come across problems, deal with them as they come. Optimise when you need to, not before. Finishing projects is hard and a problem a lot of people (myself as the worst offender) have. Great article on this here
Good luck. HTH