To write a cracking methodology I strongly recommend all my students to spend half a day or so in the library in the 'Research Methods' section. This allows for an efficient and academic development of a justified narrative to support the particularly method(s) choice being made. Dip into them like text books and seek to use definitions and explanations of strengths and weaknesses from 10-15 different sources, using 100% Harvard referencing along the way.
Thus, unless you are following a research methods course, I would not necessarily recommend that you buy a research methods book, but use many from the library. I can imagine you smile as you read this - finally a book I don't have to buy !
However, I would recommend that you read cover to cover (and this probably means buy) a book on writing a dissertation, particularly if this is your first. There are lots and lots of books of this sort out there, some of them are quite thin. I have been using "Writing Dissertations and Theses" by Murray and Beglar, which I've found to be neither thin nor intimidatingly wide, but includes a reasonable number of diagrams and seems to swiftly cover all the key topics, without over complicating things.
Pearson Education - Inside Track to Writing Dissertations and Theses