Paleobotany
Free to read here, from Project Gutenberg.
Fossil plants, Vol. 2
Fossil plants, Vol. 3
Fossil plants, Vol. 1
The Chain of Life in Geological Time
Ancient Plants
The Geological History of Plants
Links With the Past in the Plant World
Field Book: Pennsylvanian Plant Fossils of Illinois
Introduction to the Study of Palæontological Botany