Asterisk - The Future of Telephony is a great starter book.
If you want my honest opinion, find a linux distro (gentoo, centos, slackware and fedora are all popular with asterisk), install it, install asterisk, and get a dialplan working.
it will be very difficult at times, but there is so much information out there to help you LEARN what you need to know that by the end of it, you will have a fairly good understanding of the basics.
voip-info is a haven of info, and there are numerous short guides on the internet on how to get asterisk up and running. just take it slow, use google to search for any questions you have, and you will be successful.