I own all three of those books (Street Gang, Unpaved, and 40 Years), and if I had to recommend one, it would be the 40 Years book. It is currently my favorite book. However it really depends on what you are looking for. Street Gang is basically just a novel describing the creation and behind the scenes production of the show. It's a great read for the history, but can feel a little dry. Unpaved isn't nearly as informative history wise, but it is great for pictures and transcripting classic skits and songs. Definitely enjoyable if you are looking for a trip down memory lane. It can be a little on the light side though, and may leave you wanting more. 40 Years though is a nice big and heavy book with lots of picture and info about the show (history, production, actors, etc) and many things relating to the show. It is like an all inclusive, and is probably the most bang for your buck. I still find myself flipping through it a lot while the other ones just sit on my shelf (in fact I think my copy of Street Gang is packed away somewhere.).
Long story short, if you are looking for something cheaper, Street Gang or Unpaved might do you. I bought Street Gang on clearance, ad Unpaved used. But if you can afford to get 40 Years, I highly recommend it, and maybe get the other two down the road as supplements if you get a good deal. Also. Street Gang is also available as an audiobook narrated by Caroll Spinney, so you MAY be able to obtain it that way as well.