Just something I happened to stumble across...
This is by no means an entry level book, however it is free, it has a lot of reading matter that one can delve deep in to, provided you have studied the relevant mathematics this may well be a useful resource.
Book is in PostScript format, if you do not have a PostScript viewer:
GhostViewer for Windows
There are probably plenty of Linux PostScript viewers, which ever you have/find first should be fine.