OK, but not her best work
A great start, but the book never finished. I realy enjoyed the first 120 pages, but the ending appeared to be rushed (perhaps by a publisher's deadline). There were numerous story threads which were never wrapped up.
Anyway, the plot follows a team of men on two missions. The first has them journey into earth's past adopting period dress and speech as they search for alien technology that's been showing up in modern times. Quite interesting and fully fleshed out.
The second mission sends them on an alien spacecraft in a sort of allegory for travelling into the future. Everything is well done to the point they leave the alien world. After that the story is rushed and in my opinion concluded poorly.