First a small correction to Mr. Semkov's post.  There are actually two chapters on the position after 1.d4 d5 2.c4 dxc4 3.Nf3 a6 in the Chess Stars book on the Queen's Gambit Accepted by Mr. Semkov and Konstantin Sakaev, not one.  The first chapter deals with the alternatives to 4.e4, and the second chapter deals with the lines following 4.e4.  Overall, the coverage in the Chess Stars book seems fairly comprehensive and consistently strong (there is a lot of new analysis in this book you won't find anyplace else), which makes it quite useful to those who play either side of the QGA.  .

Another potential source of material on the 3...a6 lines is the correspondence games of Jonathan Penrose, who frequently employed this move order.  (Penrose was, for a period, one of the top correspondence players in the world, in addition to being a strong OTB player.)  As noted in Tim Harding's wonderful "50 Golden Chess Games" (if you don't have this book, consider buying it, you don't need to have any interest in correspondence chess to enjoy the brilliant games and fantastic annotations):  "The QGA played a crucial role in Penrose's CC career and he won with it on many occasions.  These games reveal a lot of finesses not to be found in books like Neishtadt's 1997 work on the opening."   

I think White probably does have an edge after 4.e4, as Mr. Semkov said, but in general the 3...a6 move is not nearly as bad as it might seem at first blush, and to prove an advantage White must be prepared to play some rather sharp variations.   

     - Geof Strayer

Taimanov in his 1980 book Damengambit bis Holländisch calls 4.e3 b5 5.a4 Bb7 the Haberditz Variation. He gives 6.b3! e6 7.bxc4 bxc4 Weiner-Haberditz, Vienna 1948 and according to Euwe White must play 8.Ba3!
Taimanov also gave your game 6.axb5 and evaluates it as equal.
4.e4 is the Borissenko-Furman gambit, which is rather unclear.