Complex sentence: He frequented Sky Books, a military-oriented bookstore in New York, and read many U.S. Army field manuals and technical manuals, and also credits his friend Lee Russel, a military historian, for helping him with research.
Split into simpler sentences: He frequented Sky Books, a military-oriented bookstore in New York, and read many U.S. Army field manuals and technical manuals. He credits his friend Lee Russel, a military historian, for helping him with research.