An interesting and probably worthwhile book on Theravada (or probably more correctly, the Pali suttas) and Nagarjuna is David Kalpahana's NAGARJUNA: The Philosophy of the Middle Way. This book is a translation of Nagarjuna's central text, the Mula. Tibetan type hate the book, which probably speaks well of it. The translation, while not perfect is good and the commentary is interesting, often referencing the Pali suttas.
But for me, the Theravada/Pali suttas do not need Nagarjuna.