This book is pretty outdated - Linux Music & Sound (2000) - but I think I should mention it nonetheless, because on my research for audio tools for Unix game developers I frequently came across this book.
... but let's watch for something contemporary ...
The energyXT2 music production software is a sequencer for audio & MIDI recording, similar to Logic & Cubase. The software is not free and has a price of 75$, but they offer a Linux demo. Sadly I couldn't really test it because of my sound devices, but I could start it anyway (without sound) and it looks pretty cool, and is definitely worth a try. I'll check it out on the next chance, but having any reviews would be great.
In the case you don't like using priority software and overleap, you missed a cool link about Linux VST plugins, where the creator of energyXT2 collect stuff about VST plugins for Linux, where you could also find free plugins.
I'd also like to present Hydrogen, an advanced drum machine, which aims to bring professional yet simple and intuitive pattern-based drum programming. Indeed they created a very cool program which is very close to that aim, and it's already possible to compose & arrange complex drums with lots of drum sound.
Nevertheless, the most interesting thing about music making software is the progression of Ardour's MIDI integration, what's also their project at Google's SOC.