I remember it took me forever to figure this out and the headache before learning this that it caused. Because .NET is strongly typed you are unable to access the Session, Request, or Response (and many other Http/Web object) from within a class library like you can writing in a code-behind file or on the page itself. I battled for weeks (I almost think months) trying to find ways of passing information between my pages and class library, when I finally found the answer:
Thats it. No special magic, just use the HttpContext.Current Object within a class library that is being used within the context of a website. Once I found that, it made life so much easier, and if you are reading this, I hope it makes your life easier too.