Also recommended: PENTATONIC SCALES FOR THE JAZZ-ROCK KEYBOARDIST by Jeff Burns.It is worth 10 times the price Get this book, it is so great, beyond words, Alfred Mann is like the Saint of Musical theory! What it contains is a complete study of the subject fugue - through the works of 4 theorists, Fux, modal-style, Marpurg, student of bach, Albrechtsberger, teacher of Beethoven Padre Martini, most famous theorist at the time of Mozart, and at one point the teacher of Mozart(the book contains a Mozart fugue, and the "corrections" done by Martini) Before hand it may proove essential to know the basic theory - start with "Gradus ad Parnasum"(same author) and get a good book of Functional Harmony as well. After that, this book will be all you need to start fugal writing. Good luck!
Also recommended: "Pentatonic Scales for the Jazz Rock Keyboardist" by Jeff Burns.
The good news are: If you like music, if you have studied for a long time and would like to begin with your own compositions, this is a mandatory book.