These resources are floating all over the Internet. Scanned books whose copyrights have gone out of scope. Please contribute to this list, contact me if you have any books not listed here. Download away!
A lot of these books contain information on high explosives, and are terrorist handbooks; something that this site does not endorse or focus on. The information found in these books does have use in pyrotechnics, despite the context in which it is presented. I strongly encourage people reading this material to judge the reliability of the sources of this information because some of it may be designed to injure or kill the person attempting to manufacture the outlined processes.