I'm not a mathematician by training and a rarely come in contact with mathematicians. For this reason I find solution manuals to be incredibly useful - reading them allows me to see how experienced mathematicians solve problems and I often learn as much from the solutions as the original texts. Even if I can solve I problem myself, I usually have a read through the solution to check whether there is an alternative proof I haven't thought of.
Even though a lot of textbooks have solution manuals and a lot of these manuals end up on the web, I occasionally come across some, e.g. http://terrytao.wordpress.com/2011/01/24/an-introduction-to-measure-theory/, for which it seems that either no manual has been written or the author's has done a very good job of keeping it private (I'm not saying that this is a good or a bad thing - it is none of my business). While attempting to work through these texts I keep thinking that it would be great if there was some sort collaborative online manual where individual readers could post and discuss solutions for a particular textbook. My questions then are:
Has anyone ever heard of such a thing? If so, did it work? If it did, how does it work and if it didn't, why not?
If there are any authors reading this - how would you feel about such a solution manual appearing on the web for your texts?
Is it even legal to create such a document?