Highly advanced malicious software research is my field. There is simply no other rigorous treatment of computer viruses (the somewhat hard mathematics behind what is a 'virus') in book form on the market today.
Lt. Col. Eric Filiol, a Math and CS professor and Research Scientist Officer at the Military Academies of Saint-Cyr (French equivalent of Westpoint), heads the Virology and Cryptology Lab (ESAT) in Rennes.
Prof. Filiol is an internationally renown expert on cryptology and its strong but non-obvious links to virology. He is also the editor in chief of the premier academic journal devoted to virology, Journal In Computer Virology (Springer).
The books is deep on theory fundamentals and formalization, in the French tradition of CS being subsumed by mathematics. Subsequent analysis of somewhat dated viruses (incl c source code) follows. A CD is included. Serious researchers with a background in automata theory will find this book especially useful. His second volume, Techniques virales avancees (available only in French as of May 2008) is even better.
I recommend for the aspiring researcher in order of complexity Skoudis's "Malware" (loved counterhack reloaded), then Szor's "Art and Science" (Peter, when's the update coming out?), then Aycock's book and then both books by Filiol.
Daniel Bilar