I can think of several. Interesting, both of these were early grad school experiences.
Yoram Binur's My Enemy, My Self radically altered my Jewish political identity
Marc Bloch's The Historian's Craft crystallized my historical professionalism.
Neil Gaiman's Neverwhere, and I've read Richard Adams' Watership Down probably a dozen times.
Well aside from a really good survival guide, what I'd really want are some really big multi-volume series.... An English translation of the Talmud and Mishnah, and the Durant History of Civilization. I suppose I'd settle for Abraham Cohen's Everyman's Talmud and the Durant Scientific Revolution volume.
Almost anything Douglas Adams, Terry Pratchett or David Lodge wrote. (I don't think of myself as an anglophile...)
Also Ogden Nash's poetry, Michael Bond's Paddington books and A.A. Milne's Pooh stories, but you have to read them aloud.
That's a tough one. The first one that comes to mind is the play "Mr. Roberts" -- I went through a serious play-reading stage.
I think I also cried when I read Harlan Ellison's Deathbird Stories for the first time. More than once.
Susannah Clarke's Jonathan Strange and Mr. Norell
In my first year of graduate school, I read a book which was almost exactly the book which I had in my head; it was better than I could have done at that point, better than I could do now.
A Social History of Banking in Modern Japan
The Impeachment of George W. Bush and Collapse of the Republican Party
The Protocols of the Elders of Zion
The Book of Revelations.
Neal Stephenson's Baroque Cycle trilogy
The Oxford Dictionary of Quotations.
Thomas Sowell, Migrations and Cultures
Kyle Ward, History in the Making: An Absorbing Look at How American History Has Changed in the Telling over the Last 200 Years
Now tag five people: No. But I will note the bloggers which I read who have done this meme already, many of whom have also declined to tag: Another Damned Medievalist, Terry