I would actually spend the money to buy Thinking with Type, if you're really serious about learning typography. Having the tree version lets you get really up close and personal with the various types, and it's easy to flip between pages to compare letterforms, etc. Another one to check out is Elements of Typographic Style [1].
[1] http://thinkingwithtype.com/
[2] http://www.perceptualedge.com/blog/?p=1374