I just discovered this nice book that was published this year

 - [Gordon and McNulty (2012). Matroids. A geometric Introduction.][1]

It is an undergraduate textbook on matroids! (Maybe the first undergraduate one completely devoted to matroids?) And from what I could see from the first few chapters, it is highly pedagogical and very well written.

  [1]: http://www.amazon.com/Matroids-Geometric-Introduction-Gary-Gordon/dp/0521767245/