There are several guidebooks that cover Tajikistan, but none of them pay much attention to the south, hence the reason for this website.
The book that dedicates the most pages to the south (Khatlon Province), is Tajikistan and the High Pamirs, by Robert Middleton and Huw Thomas (click here for more information). However, it is just 16 pages out of 700 pages total. This book is, however, great for the rest of the country. And in addition to the usual practical information, it provides endless hours of reading on culture, history, wildlife, botany, etc… We strongly recommend buying a copy.
Lonely Planet’s latest (2014) edition of its Central Asia guidebook covers all of Central Asia, and the Tajikistan section does not provide information on southern Tajikistan. But it is worth the purchase if you want to research the rest of the country. Bradt has recently (2013) published a guide to Tajikistan, and it has a short 12-page chapter on Khatlon Province.