Modulation is the art of smoothly moving from one musical key to another. Modulation is one of the ways to make your music more interesting. If this sounds intriguing and you know how to read music notes, stay with me.
Three weeks ago, I set out to learn something about modulation. I took notes while doing so, and I'm making those notes available for anyone interested in the subject. The notes so far encompass 257 (!) pages full of modulations between different keys. Each modulation each time is written out in all keys, leading to a catalogue of modulations from any key to any key (well... almost. It's a work in progress). It also contains brief explanations about the underlying principles that make the modulation work. Using those principles, it should be possible to create new modulations yourself.
Most of the material is illustrated in the context of classical music theory (but all the underlying principles illustrated there apply also to jazz/blues/pop/gospel/...), but part III also contains some beginnings of Jazz and Gospel cadenzas.
You can also download a prebuilt .pdf from the files section.
If you have downloaded and skimmed the book, be sure to comment either here or on the book's forum.