I have some audiobooks on my phone, but I don't want to hear these when I play "All music". Is it possible to exclude some folders from getting added to the library? What are the file types that automatically get added?
You should be able to create a file called ".nomedia" (without quotes) in the folder you don't want scanned. It is supposed to tell the media scanner not to look in there.
Using Astro (available in the market), you should be able to create a folder on your SD card. Prefix the name of the folder with a dot (".") and the folder will be considered hidden in the Linux/Android file system. This will keep it from being included in the library.