I have read most of Janette Oke's books, and I must say that "When Calls the Heart" is really one of her best books. It is one of the few Janette Oke books I have actually enjoyed, without getting annoyed at the characters or finding historical inaccuracies.
The book's heroine, Elizabeth, travels to the western frontier to be a teacher. Her adventures are interesting and often humorous (but not outlandish) as she adapts from city life to frontier life. Of course, she meets a handsome Mountie and falls in love with him... Untypically, Janette Oke doesn't have them meet each other right away, and quickly marry them off; she allows Elizabeth to remain in suspense for quite some time and get to actually know Wynn (the Mountie) better. That is one thing I liked better about this book than other Janette Oke books- the characters were more realistic (on the whole) and they did not fall in love at first sight and want to get married right away. I liked that because it left the plot freer for other topics besides the characters' romance.
I must say, however, that one must not let the good reviews of "When Calls the Heart" to lead them into buying the other three books in the "Canadian West" series. They are not nearly as good as the first book. In fact, I highly suggest that you read "Mrs. Mike", by Benedict and Nancy Freedman, instead. "Mrs. Mike" is a true story, and when you compare it with the remainder of the Canadian West books, it is obvious where Janette Oke got much of her material for incidents and characters.
You'll enjoy "When Calls the Heart." I definitely did. The other three books aren't as good, but you can read "When Calls the Heart" by itself; it doesn't necessarily need sequels.