Dream; or, The Simian Maid — S. Fowler Wright

Marguerite travels through time to become Rita, a simian-woman of the Tree People tens of thousands of years ago. Together with Stele, Elysa, Prince Thelmo, and Princess Tekla, she must fight the invasion of the semi-intelligent rat people who threaten to exterminate the young human race. The first novel in the Marguerite Cranleigh fantasy series.