Have to agree - once more - with Paul Secor's assessment of the blues books situation.
And second BBS's recommendation of Demetre and Chauvard's 'Voyage au Pays du Blues'! Fascinating. The two were old friends who introduced me to most of the blues masters when I was still in highschool...
Another good introduction to the albums of the blues singers is Frank Scott's 'The Down Home Guide to the Blues' which seems easy (and cheap) to find:
http://www.amazon.com/The-Down-Home-Guide-Blues/dp/1556521308
Paul Oiver is a must read but you should also look for his JSP compilation 'The Meaning of the Blues':