The first question which people tend to ask about ALSA is why they should use it over the sound drivers included in the kernel - there are several reasons. Firstly, the ALSA drivers support more sound cards than those in the kernel. Secondly, the OSS emulation is in some cases faster and better than the original OSS driver itself. And finally, there are some programs which can use ALSA's enhanced features to better drive the soundcard.
ALSA is also likely to be the future of Linux Sound (hence the name Advanced Linux Sound Architecture), and the ALSA drivers will probably be included into the main linux kernel at some point during the 2.5 development series, leading eventually to ALSA being the "standard" sound drivers into the future 2.6 stable kernel.
The following five sections of the book deal with the five separate components of alsa: the drivers, the libraries, the utilities, the tools and the OSS compatibility libraries.