Knowledge of George Orwell’s Nineteen Eighty-Four and Animal Farm is generally assumed. (The rest of his output, not so much.)
The **Masters of Rome **series by **Colleen McCullough **is consistently held up as one of the best historical novels series. Meticulously researched, it brings the transition from Republic to Empire to life.
**The Professor and the Madman **and **Longitude **are held up as great narrative histories - PatM is about the Oxford English Dictionaries creation but diverges to discuss dictionaries in general, Johnson’s dictionary, Oxford, the players involved - all wrapped up in a story about one of the big contributors, a crazy American doctor. Longitude does the same thing for the history of discovering a reliable way to calc longitude while at sea…
To Kill a Mockingbird
Watchmen
Malcolm Gladwell’s books - and I would add How We Decide by Jonah Lehrer
Adams had some gut-busting funny lines, but they are very sparse. Also, to enjoy the books, you really have to see the TV series; Adams owes Simon Jones big time, since he managed to make the jokes much funnier than the are in print.
As for books, Heinlein seems required for science fiction. Stranger in a Strange Land is assumed.
We also mention what could be called anti-books quite a bit as well:
Flowers in the Attic
Valley of Horses
And don’t get me started on Muammar Qadaffi . . . ermm . . . Khadafi . . . Gaddaffi ? . . . the Libyan guy.
I’ve not only read nothing mentioned in the OP, many of the following posts are full of books I’ve never even *heard *of. I guess I flunk Dope.
In re. Delany, I’ve read Dhalgren three times.
I’ve heard of most of this stuff, but not read much or worse, I started to read it and it defeated me.
LoTR has been mentioned, but it’s worth pointing out that Silmarillion seems to be a men/boys dividing line around here.
“A People’s History of the United States” by Howard Zinn.
One I’d like to add to our repository: “The Sociopath Next Door.”
Also just as a reference book, Strunk’s The Elements of Style, or something similar.
I would think something by Phillip K. Dick should be on the list, and Harlan Ellison, too.
The OP is asking about books most often mentioned at the board, not your favourite books. I get the impression some people are mentioning their favouriate books.
So, in what is the spirit of the OP, I’ll say:
Some of the Straight Dope books
The Bible
<raises hand fervently> Boy here!
(I’d rather learn double entry book-keeping than try to read The Silmarillion again. DEBK has to be less tedious and might come in useful some day)
I’ll throw in Shakespeare, just 'cause. And Freud, too.
I agree.
For required SD reading, I’d agree with Atlas Shrugged, the Bible, Guns Germs and Steel, 1984, The God Delusion and LOTR. The rest are popular, but I don’t think they come up wth the same frequency as the ones I just listed
The Barn House.
Twilight, as a sort of “stupid book” reference guide. 
Going Rogue
just kidding
Terry Pratchett is required reading across the internet. I guess maybe internet people=computer geeks=speculative fiction geeks? I definately see his name come up on the screen a lot more than I do IRL.
As for self help books, Allan Carr’s Easy way to stop smoking and the book "If you meet the buddha in the road, kill him" are mentioned often.
English Passengers by Matthew Kneale!!
I learned of it here and LOVED it, and have seen it recommended over and over. Rightfully so - the book is amazing.
I think** The Da Vinci Code** qualifies. It seems that everyone here has read at least part of this and taken a few shots at it. Putting it down in disgust before the end of the second chapter is quite fashionable.
Not to mention that it makes a good stand-in for all that is wrong with American literacy, etc. in this day & age.