Here is the third of my one-book-at-a-time bookshelf.
Unlike many current fantasy writers Robin McKinley hasn't produced an extended series of books. I will talk more about my view on series another time, but there are advantages to limiting one's output. In the case of The Hero and The Crown it is a prequel to the Blue Sword but clearly it isn't an afterthought type of prequel but rather a completing of the story and in my opinion there are good reasons for reading them in the published order. The Blue Sword wouldn't work as well if you had all the background from this book, and similarly this book wouldn't work as well without familiarity gained from the first book. They are a nice pairing and I commend them to you. For your bookshelf.