The information that I find important when understanding the requirements of a software system or component include:
Karl Wiegers, author of Software Requirements and More About Software Requirements also has a website called Process Impact. Although the templates are for sale, you should check out the Goodies section, which has documents that are released for the nominal sum of $5.
If you have the money, I would recommend both books. If you don't want them, maybe your company can buy and own them if there is a company library or something like that. Either way, they are must-reads for anyone working on requirements gathering/analysis.
I also have to recommend not only the book that Donut suggested in his last sentence, Software Requirements (2nd Edition), but also a companion to that book - More About Software Requirements. Both books are by Karl Wiegers, and his website has some interesting reading as well, in particular the Goodies section, which has freeware sample documents and templates.