This is, to the best of my knowledge, the only guidebook dedicated solely to Tajikistan. It's written by a couple of Westerners who lived in Tajikistan for years and obviously love it. It's packed with all sorts of interesting information, much more than you'd ever find in any sort of Lonely Planet (certainly more than the Central Asia one, since it has 6 countries in a book that's much smaller than this one), but it's also not quite as practical as LP. You probably need both.
Half this book is about the Pamirs, and especially about the Russians and other Europeans who explored them. That's not my favorite part of Central Asian history, but there's plenty in here besides that to make the reading worthwhile. We're hoping to be in Tajikistan in about a year, and I already have a list of places I want to see.
A new edition is set to be published in a few months; wait till then if you can to buy it.