Language Resource Site
Books Ngram Viewer is a website you need to bookmark. You just type any word or phrase in the Search box, and it will show you whether people in previous eras used it.
This will certainly be a research boon for historical writers. There's nothing worse than reading a historical novel and a character says something that you just know wasn't uttered in the time era of the novel.
Always strive for accuracy in your writing, especially when it comes to historical references.