Strangely enough, the first book that popped into my head was 'Dracula' by Bram Stoker. I've mentioned before that I've read this... an uncountable number of times... but for sure, it's been more than 50. I was an early reader (yes, dyslexic people can be early readers) and quickly skipped from Dr. Seuss to chapter books like 'Riverboat Adventures' by Lucy Kincaid. If you have a young child, this really is an amazing book if you can find it... my nephew has loved it ever since he was three (he is my little clone after all, right down to books, food and obsessions).
Since my mother was a literature major in university and my father a philosophy major, there was always a hoard of interesting books on the shelf, and because I was a voracious reader, I breezed through 'The Hobbit', 'Frankenstein', 'Dracula' along with Edgar Allen Poe and Mark Twain before I was ten. Though I didn't really like 'The Hobbit', 'Dracula' and anything by Edgar Allen Poe (most notably, 'The Pit and the Pendulum' since that was the first of his I read) still bring me a warm and fuzzy feeling... yes, I know how strange that sounds.
Another two books I would consider comfort books would be 'Dr. Doolittle' by Hugh Lofting and '20,000 Leagues Under the Sea' by Jules Verne. Right now, they are at my parents' house, awaiting my little nephew's discovery.
As for grown-up books, the ones I return to again and again are: 'Veronica' and 'A Trip to the Stars' by Nicholas Christopher, 'The Solitaire Mystery' by Jostein Gaarder, 'Bliss Street' and 'Too Small for Basketball' by Kris Kenway, 'Angel in the House' by Kate O'Riordan, 'Beachcombing for a Shipwrecked God' and 'A Pocket Full of Names' by Joe Coomer, and 'Amrita' (and almost anything else) by Banana Yoshimoto, which also fits under the books that changed my life category.
Of the YA books I've read recently, 'Plain Kate' by Erin Bow will definitely become a comfort book I read again and again.
What are your comfort books? Are they ones from your childhood, teenage years or adulthood? Is it because of the story, or do they coincide with major events/memories?