Hmm.. I would recommend a few books! Some of the ones I have taught are amazing reads for any age though I do admit, most of my casual reading is young adult literature because I'm either researching books for my units or the fact that they are fun, easy and usually great reads.
I love Edith Wharton and I would recommend Custom of the Country and The House of Mirth. They're not the happiest of books... but they're amazing in their complexity and ability to make you think.
I just finished a series by Scott Westerfeld after three students did book reports on it. Book one is called "Pretties," book two "Uglies," book three "Specials" and book four "Extras." It is an amazing series; yes written for young adults but it's awesome. It's all about a society in the future where people on their 16th birthdays are forced into having drastic plastic surgery to make them beautiful; the theory being that beautiful people have nothing to argue about, start wars about or generally care about at all but what if you don't want to be pretty? It's a really great series, I highly recommend it.
I'll chime in with Harry Potter too! I think book 7 jumped the shark, so to say and kind of insulted readers but I love the series too much to snark it much. Along those same lines, the Artemis Fowl series is pretty good too, it's kind of an Anti-Hero Harry Potter series and I like that a lot about it.
For the classics besides Wharton, I love Lewis Carroll, C. S. Lewis and T. S. Elliot... But I might just like abbreviations
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Anyway, I have typed longer than I bet you want to read! Sorry for going off, I just love books. Kind of have to in my line of work, lol. Hope you find something you want to read!