Jayne — Miller's Collector's Guides are relatively inexpensive and generally accurate, so are a good starting point. Ivo Haanstra's Glass fact file a–z (yes, our Ivo) is indispensable and inexpensive. Other than these I suggest you spend some time at Broadfield House Glass Museum, where you can browse around most of what books and journals are in print.
If you want to buy anything pricey, borrow it from your library first and cuddle it for a couple of weeks. It might cost you a pound or two for interlibrary loan, but that's better than spending considerably more and regretting it afterwards. Always remember to check any expensive book for missing pages and plates before you leave the library — for example there is one copy of Ysart Glass in BL's interlibrary loan stock that has pages missing.