In my opinion, the best Mustang book I've seen {and I must have 40-50 of them} is 'Mustang Chronicle' by Jerry Heasley. I first saw this book when a friend bought a new Mustang in '99, and Ford sent out welcome packages containing a key fob, I think a leather bound owner's manual, and this book. My friend at first gave it to me, {he's not a car guy at all} but I foolishly kept it on my desk at work and one day he came in and said if I was done with it he'd take it back. By a strange coincidence, a few years later my BIL, who's not a car guy either, picked up a copy on a clearance table in a bookstore and gave it to me as a birthday gift.
This is a really good book, with lots of color pictures of many different models. The trouble is, this book is out of print and you'll never find it in a book store these days. Hopefully it will go into another printing.
Amazon has a few used copies, and they are dirt cheap. I'd highly recommend getting a copy, especially at these prices. As I said in another post, I just bought a used book from Amazon's used book system and I am very satified.
Here's a direct link to the book:
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'70 Mustang Mach 1 351C 4V/FMX/3.25 Open
'70 Mustang Convertible 250 I6/3 speed/2.79 Open
'72 Mustang Sprint Hardtop 351C 4V/FMX/4.30 Trac Loc
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip