Ow. No, that's not part of the advice.
- Editions, editions, editions. Literary criticism is temporary, but the literature itself is forever (one hopes). If at all possible, buy the best scholarly editions available; if not at all possible, given how much scholarly editions can cost these days, buy the nearest possible thing. For anyone doing 18th and 19th c. literature, Oxford World's Classics and Penguin Classics are your friends; so are Broadview (reprints of rare novels and poetry collections) and Kentucky (18th-c. women's fiction). AMS, Scolars Press, Arno, and Woodstock all offer useful facsimile editions, but can be exceptionally expensive. When in secondhand bookstores, keep an eye out for old Virago reprints (mostly 19th-c.). Everyman and Pandora reprints should be treated with some caution, as they can suffer from typo-itis.
- Anthologize. Buy up the period anthologies from Oxford, Penguin, and Blackwell. Look for useful old anthologies, like Norton's two-volume set of 17th-c. verse. Be on the alert for editors who spend their lives doing anthologies--for example, Peter Haining, who has assembled several volumes devoted to 19th-c. mystery and horror.
- Be referential. Load up on Oxford companions to whatever, the Cambridge companion series, dictionaries of literary and rhetorical terms, and biographical dictionaries. Learn how to spot-check entries.
- Don't buy literary criticism or theory until you're ready to specialize. Seriously. If you're going to be lugging books around from one state to another, you want them to be relevant to your work. Once you've identified your Ph.D. fields, start buying general works in those areas. Put off the most specialized purchases until you begin writing your dissertation. Ask yourself how many times you plan on using a given book--or, if you can only access it via interlibrary loan, how long you're going to need it.
- Is there a library in the house? Adjust your purchasing habits to the strengths and weaknesses of your own campus' library. At a small campus, you may find yourself needing to buy everything in your area of interest.
(Owww. Did I say that before? And I've still got seven bookcases to go...)