Donald Jay Grout's A History of Western Music introduces both Greek and medieval modes. Lee Evans's Modes and Their Use in Jazz is both comprehensive and accessible for any musician who wants to begin to study that subject. For Western musicians, an introduction to ragas, that is neither too vague nor too technical, does not seem to be available as of this writing.
Available under Creative Commons-ShareAlike 4.0 International License. Download for free at http://firstname.lastname@example.org