News from Tartary by Peter Fleming is the rest of the story of Ella Maillart's Forbidden Journey. Fleming travelled the entire distance with Maillart and wrote his own version of their trip. Both books are excellent, especially since Maillart and Fleming write about different things, but clearly travel well together.
Sadly, the edition I read doesn't have any of the photos Fleming mentions; it doesn't even have a map so unless you're pretty familiar with the path they travelled, you're either going to have find your own map or just wander along with Maillart and Fleming. Or find an older edition because it was rather silly to cut out the pictures.