If the focus of your enquiry is the effectiveness of a new policy within your area, for example, on learning and teaching in schools, it will be important to understand and analyse policy documents as well as any other relevant official publications. Very often, reading one article identifies further sources which you feel you must pursue. Take care however, as searching the literature can take on the characteristics of a snowball rolling down hill - growing steadily larger until you are buried under a mass of printed material. Self-discipline is required as temptation will come your way, but keep reminding yourself of the main themes you identified. The aims of your enquiry should appear to emerge, almost naturally, from your consideration of the literature - the issues raised, assertions made, omissions and differences of opinion, should inform their development and these aims, in turn, should be reflected in the questions which you establish for the enquiry.
Sundry advice Go to the library before the bookshop.
Books are dear and you should only buy those which you are sure you will find of fundamental importance in the longer term. However, some libraries may not have the most recent texts in your field and you should look further afield. Make a friend of the librarian. Most librarians are only too pleased to help and many libraries, particularly academic ones, produce leaflets to help readers negotiate their way through the books, journals and other publications stocked. Collect such leaflets and read them.