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TeX and LaTeX 
Making TeX Work (by Norman Walsh) [Mirror] TeX by Topic (by Victor Eijkhout )
A Beginner's Introduction to Typesetting with LaTeX (by Peter Flynn, in PDF)
A Gentle Intorudction to TeX (Machael Doob) Getting Started with LaTeX (David R. Wilkins)
Math into LaTex: An Introduction to LaTeX and AMS-LateX (George Gratzer, in PDF)
The teTeX HOWTO: The Linux TeX Guide (Robert Kiesling) A Simplified Introduction to LaTeX (Harvey Greenberg)