This site in general is good for theory and background and if you go to the index then to section 16 they have a lot of recommended kits.
http://ldsg.snippets.org/index.php3
There are quite a few very good diy kits out there but there are a lot more that are not so good. I can recommend Northcreek kits based on experience. it has been a few years but they are very good
http://www.northcreekmusic.com
Also a nice two way from Ellis audio that I haev heard and is very good
http://www.ellisaudio.com/
A fairly new TL design from Madisound that I have not heard but has had lots of good stuff said about it:
http://www.madisound.com/thor.html
some basic diy info:
http://www.loudspeakers101.com/Sound.htm