I have just finished reading this, and it's an excellent book.
It goes into much more detail than I have seen before about what happened before,during and after the invasion. It begins in 1809 with Shah Sujah on the throne for the first time, and follows the story through his fall, the rule of Dost Mohammed, the invasion itself, the occupation and what happened during and after the retreat. Despite the length (just over 500 pages in hardback) and the amount of detail, though, it's a very easy book to read. The use of the poetry of the time is very good, and does illustrate what happened.
There are also several parallels between the 1840s and now. It quotes the chilling comment by Merab Khan of Qalat to Alexander Burnes in 1839: "You have brought an army into the country", he said. "But how do you propose to take it out again?"
I have read a number of books about the various Afghanistan wars, and enjoyed them all (including, of course "Flashman" by George MacDonald Frazer). This is, though, the best one, and I would recommend it to anyone.