As suggested by tmarshallva you should ask whoever is responsible for the overall development effort) and ask what other documents you should review for requirements.
If the requirements are unclear or in case you need to make your own guesses for requirement (in case the documents are not available) kindly confirm those with the client before proceeding for test case creation.
-> For the Requirements Gathering first try to get as much information as possible from the 'Concept notes' written for that business requirement.
-> Then try thinking on why this requirement had come? and Who are the users behind that, i mean who will be benifitted from that.
-> Then talk to the Product Manager who is dealing with the requirement. By doing this you can get an overview on why this has come and how is the solution direction they are taking.
-> If possible you can also discuss with the Customer or a presales guy(incase if talking to the customer directly is not possible) who has bought in the business requirement to understand the customer perspective of it.
In Case if you are to write a testcases for a generic requirement like eg. i18n (Internationalization) support or Multi browser support etc. which are common features all around you can just get as much info as possible by googling about it.