Here are a few mathematics resources that I have found useful.
Introduction to Complex Analysis: WWL Chen
http://www.maths.mq.edu.au/~wchen/lnicafolder/lnica.html
This is very clearly written and better than most books I have read. The explanations and examples really stand out.
Complex analysis and Functional Analysis: Douglas N. Arnold.
http://www.ima.umn.edu/~arnold/502.s97/complex.pdf
http://www.ima.umn.edu/~arnold/502.s97/functional.pdf
2 excellent documents. The first is probably a little more accessible than the second which requires a substantial mathematics background.
A First Course in Complex Analysis: Beck, Marchesi and Pixton
http://math.sfsu.edu/beck/papers/complex.pdf
First Course in Linear Algebra
http://linear.ups.edu/download.html
Cauchy-Schwarz Master Class: by J. Michael Steele
Whilst this book is not free, several sample chapters are downloadable (http://www-stat.wharton.upenn.edu/~steele/Publications/Books/CSMC/CSMC_index.html ). A truly remarkable book whose clarity and insight is a rarity amongst such books, which often gloss over details and leave the reader baffled. Although a specialised work, I have come across very few mathematics books that can equal it as an educational tool. If you are about to embark on a postgraduate Mathematics (or indeed undergraduate) course of study make sure you read this book.
List of Free Mathematics Books: a great resource!
http://www.math.gatech.edu/~cain/textbooks/onlinebooks.html
This should be your first port of call if you are looking for free online material.
There is another list of free online Textbooks here: http://linear.ups.edu/opentexts.html
Introduction to Tensor Calculus and Continuum Mechanics
http://www.math.odu.edu/~jhh/bookpdf.zip
by John H. Heinbockel
Concrete Mathemathics, 2nd Edition. (aimed at CS postgraduate students)
http://www.amazon.com/Concrete-Mathematics-Foundation-Computer-Science/dp/0201558025
Not a free book, but if you are about to pursue Computer Science postgraduate studies in algorithmics, complexity analysis or a related subject, then this work is required reading.
If you know of any other high quality work, free or otherwise, please let me know.