Looking for genre classics

While the latest hype-trains and the guaranteed oldies give me a reading list a few thousand books long, I like to read things which are left by the wayside. This list here is a good example. The author gives a list of genre classics. Books which aren't good enough to make the top 1000 books of all time, but are classics in their own genre and influenced a lot of future authors. The Princess and the Goblin is a good example. Everyone interested in Tolkien and the Inklings has read it, as well as those who like modern fairy tales, but it doesn't crop up much in recommendations lists. These are books which aren't quite as commonly discussed, but still good and important for people interested in the genre.

So, if you have a favorite genre or sub-genre I would love to read your 'genre classics' list, with maybe a sentence about why I should enjoy it. Not quite as comprehensive as a class on books, more than a bullet point.

Edit:
I just realized I didn't change the title. By the 'gap', I originally meant the gap between the books everyone suggests from the past and the mountain of dredged pulp you find in libraries and bookstores: books which are worth still reading, even if they aren't one of the 'Classics'. More like underrated recommendations.