I have recently (in my late 30's) become a huge fan of John Bellairs' gothic novels for young readers. I love the 1950's settings and the consistent story-world that fuses the boy detective genre with the occult and the supernatural. I also love how the stories often feature the theme of friendship (and the conflicts of friendship in boyhood) in a touching and sweet way. A bunch of novels in the series were completed by Brad Strickland when Strickland took over the series after Bellairs' death. I also enjoy Strickland's original stories in the series.
Another distinctive feature of this wonderful series is the series of covers by Edward Gorey.
Like a lot of people, I suspect, I became aware of Gorey through the animated introduction to the PBS Mystery show. After that, I discovered his wonderfully twisted Gashlycrumb Tinies and a variety of wonderfully sinister, mysterious, and humorous books.
His covers for the Bellairs series do a wonderful job creating that sense of mystery, gothic creepiness, and wonder that reflects the series of stories. The images also tell a little bit of the story in a way that hints at the mystery and makes me want to find out more. They are very odd and intriguing.
Someday I want to embark upon collecting all of these hardcovers with the beautiful wraparound dust jackets. Until then, I can admire the images on a wonderful blog that is dedicated to the work of Gorey: