I like Ackey and Gulanick: Nursing Care Plans, 7th Edition
I used the Gulanick book as a supplement to (substitute for) the book required by my nursing program. The syllabi didn't tell us which book we had to get most of our information from, just that we had to document the sources and use our required care plan book for at least one of the nursing diagnoses in our papers for clinical. I found the Gulanick book more user friendly than then book we were required.
If you don't have the NANDA-I 2012-2014 (most current issue, free 2-day-shipping for students from Amazon) you don't have the definitive book on nursing diagnoses. It's a paperback, not expensive. You will use this in conjunction with whatever other book you get.
No nursing diagnosis, no nursing plan of care. Seriously. Even if your faculty forgot to put it on your bookstore list, get it anyway and fake 'em out when you know better than anyone.
Word to the wise, you and every other nursing student: If the nursing diagnosis you're in love with isn't in this book, it isn't an approved nursing diagnosis and you cannot put it in a plan of care. Reading the diagnoses and the defining characteristics for them will give you a huge leg up on your classmates when it comes to learning to think like a nurse, the hardest thing a student has to learn.