Linear Time-Invariant Systems and Convolution. 

A gentle, tough quite formal, introduction to the topic. By Wilson J. Rugh, Johns Hopkins University, Baltimore.

 Dynamical Systems on Wikipedia

Wikipedia is an on-line Encyclopedia made by its own readers (you included, if you want)

New Possibilities in Sound Analysis and Synthesis - Xavier Rodet, Philippe Depalle, Guillermo Garcia (IRCAM)

A basic article about the  "FFT-1" method for additive synthesis 

CCRMA

The page of  John Orion Smith III at Center of Computer Research In Music and Acoustics -  Stanford (CA). Here the huge amount of links of papers and pages about synthesis and signal processing.

CNMAT

 Adrian Freed page at Center for New Music and Audio Technologies - Berkley.

Musical Signal Processing - Edited by: Curtis Roads, Stephen Pope, Aldo Piccialli (+), and Giovanni De Poli

Short on line version of the paper book.

The Scientist and Engineer's Guide to Digital Signal Processing - by Steven W. Smith, Ph.D. California Technical Publishing ISBN 0-9660176-3-3 (1997)
A whole 640 pages book fully downloadable