The Canterbury Tales is a pretty impressive collection of 24 tales in total. Together, they make 17K lines written in Middle English, so, don't be surprised if you see a few (or, rather, a lot of) strange words and "wrong" spelling. Geoffrey Chaucer started working on his monumental book, his most famous and critically acclaimed work, back in 1389. The poems are mostly written in verse, but there are some that are in prose.
By the way, it's worth mentioning that every single story is presented as a small part of a tale-telling "contest" between several pilgrims who travel from London all the way to Canterbury. So, what's the prize, what would the winner get? A free meal at an Inn on their way back! Now, this isn't the first book by Chaucer. He wrote quite a few bestselling novels and poems before he moved on to The Canterbury Tales.
The critics claim that he used these stories and the characters in them to paint a satiric, ironic and judging portrait of the English way of life back in the 14th century. The Church took a lot of the "beating" - the author had some strong words for them. And given the fact that the collection is pretty big, Chaucer managed to write about a very wide range of social classes and types.
Nobody did that in English before him, and that is why he's considered to be one of the greatest writers of all times. Yes, the characters are all fictional, but they do have their real "prototypes" and allow the readers to learn about the strange customs, traditions, and practices of that time. The Canterbury Tales is, without a doubt, an outstanding book that gives unprecedented insight into the lives of the English folks in the 14th century.