Ludwig van Beethoven 
Louis Van Beethoven’s Studies in Thorough-Bass, Counterpoint and the Art of Scientific Composition [PDF ebook] 
Collected From the Autograph Posthumous Manuscripts of the Great Composer, and First Published, Together With Biographical Notices

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. I have little to add to these remarks; the genuine ness of the Studies is unquestionable, and, apart from the collateral interest attached to them, they are of great intrinsic value to all young musicians who desire to get at the kernel without unnecessary difficulty in breaking the shell.