Professor David Anthony Santos was a professor of mathematics at the Community College of Philadelphia. Professor Santos had apparently an accident and he is no longer with us since 2011 but his work is. He wrote several textbooks in Latex. Thanks to Professor Eleftherios Gkioulekas, the collection of the source files has been saved (these can be downloaded here.) Some interesting blog which mentions about Dr. Santos as a teacher is linked here. I have updated the Linear Algebra textbook (a few minor changes/ corrections) and the source file can be downloaded at this link.