The guidebook description mentions that only the hearthstone of the house can be seen, so the deteriorating structure in the photo may not be the Kelsey House. But it's in the general area.
From the Guidebook:
Nancy Kelsey arrived in California in 1841 with the Bidwell-Bartleson party and settled with her family south of present-day Calistoga. Now the hearthstone is all that can be seen of the house. The property is owned by the Rockstrohs.