Linear Algebra, Infinite Dimensions, and Maple  
Linear Algebra, Infinite Dimensions, and Maple can be downloaded free of charge in pdf format. 
Derivations of Applied Mathematics  
Free book Derivations of Applied Mathematics is a book of applied mathematical proofs. If you have seen a mathematical result, if you want to know why the result is so, you can look for the proof here. 
Inverse Problem Theory and Model Parameter Estimation  
The use of actual observations to infer the properties of a model is an inverse problem, which are often difficult as they may not have a unique solution. This book proposes a general approach that is valid for linear as well as for nonlinear problems. The philosophy is essentially probabilistic and allows the reader to understand the basic difficulties appearing in the resolution of inverse problems. 
Directions in Infinite Graph Theory and Combinatorics  
This free online book consists of invited surveys of various fields of infinite graph theory and combinatorics, as well as a few research articles. It aims to give some indication of the variety of problems and methods found in this area, but also to help identify what may be seen as its typical features, placing it somewhere between finite graph theory on the one hand and logic and set theory on the other. 
Mathematical Tools for Physics  
This book is for the undergraduate level, providing a onesemester bridge between some of the introductory math courses and the physics courses in which we expect to use the mathematics. This is the course typically called Mathematical Methods in Physics. 
