My first instinct is to say " The Naked Civil Servant" by Quentin Crisp. I won't presume to believe that everyone would enjoy it but it has been quite haunting to me. The sort of book that just gets better and better in retrospect.
When a French expedition in Antarctica reveals the ruins of a 900,000 year old civilization, scientists from all over the world flock to the site to help explore and understand. The entire planet watches via global satellite television, mesmerized, as the explorers uncover a chamber in which a man and a woman have been in suspended animation since, as the French title suggests, 'the night of time'. The woman, Eléa, is awakened, and through a translating machine she tells the story of her world, herself and her husband Paikan, and how war destroyed her civilization. She also hints at an incredibly advanced knowledge that her still-dormant companion possesses (who is not her love Paikan, but the scientist Coban, whom she hates), knowledge that could give energy and food to all humans at no cost. But the superpowers of the world are not ready to let Eléa's secrets spread, and show that, 900,000 years and an apocalypse later, mankind has not grown up and is ready to make the same mistakes again.
The Count of Monte Cristo - Alexandre Dumas. I know it's a classic and that everyone here will have heard of it, but it still gets overlooked by people. Either they're put off reading classics by being forced to read them in school, or they go for more mainstream titles by Dickens or the Bronte sisters. Well, this book is an absolute treasure from start to finish. I know it looks long and daunting, but this book has everything. A tale of love, revenge, wrongful imprisonment, hidden treasure, sword fights, murder, the works.
And don't be put off if you saw the film and thought it was a bit pants. They never manage to do this book justice. The version I've linked to has the true story that it was (very loosely) based on in the introduction, and all I can say is crikey. Admittedly some of the characters are a little one dimensional, and it's also a little wordy (he was paid by the word afterall), but they're good words damnit.
I love adventure books and this book is one of the best adventures ever. Dumas knew how to buckle a swash. Truly a master of the genre and one of the few books named a classic and actually worthy of the accolade.