The recommended "pre-study" books that we bought the summer before nursing school started were: Nutrition and Diet Therapy, by Lutz and Przytulski, some Medical Terminology
workbook that I enjoyed very much, and a dosage calculation book that I should have worked on a lot more. That was enough to keep us busy for a couple months.
Supplemental reading to expand your knowledge base is good, but if you've got cash, you might want to also think about your reference library that you're gonna need. Suggestions for that would be a drug book for nurses, (try to get one the year school starts, so it won't be out of date, Davis's is good, Springhouse has been known to leave some drugs out), a good medical dictionary, a nursing diagnosis book
(I have one by Ackley and Ladwig, published by Mosby) and a book on laboratory and diagnositic procedures. (so you know how to read all those lab results)
None of those are really "supplemental reading" but they would all come in handy when you start, and here in my school, those are not part of the textbooks, you just should have them yourself. They are in the library but that doesn't help me at two AM in the middle of a care plan.