Interesting that nobody has yet recommended Stephen Baxter. Most of his work is standalone novels, though several of those fit into a sort of overarching universe/future history dubbed the 'Xeelee Sequence.' Much of his work is hard SF, but takes the weird ideas of modern physics and spins universe-spanning consequences from them- some of the ideas are quite mind-blowing and seem less "hard sci-fi" and more "transcendent fantasy" to me. Admittedly, most of his novels are fairly long (on the order of 600 pages or so). That said, I'll recommend "The Time Ships" (essentially a sequel to H.G. Wells's short story "The Time Machine") and then let the reader make up his/her own mind on the author.
He's even done a few collaborations recently with Arthur C. Clarke, which worked out quite well I thought. Two of them are a dualogy called the "Time Odyssey," but each novel is essentially standalone without much impact on the other. Those two are "Time's Eye" (the first one) and "Sunstorm" (the second). Before doing the dualogy, they wrote a nice short novel titled "The Light of Other Days" which is one of the most inspirational books I've read in years. All three of these books are near-future and could be termed hard SF, though they don't explore the underlying science as deeply as many entries in that category do.
And I have to second the recommendation for Douglas Adams- "Hitchhiker's Guide to the Galaxy" is a phenomenally good and funny book, as are its sequels "The Restaurant at the End of the Universe," "Life, the Universe, and Everything," and "So Long and Thanks for All the Fish." There's also a fifth book in the series, "Mostly Harmless," but personally I found that one a bit nihilistic and depressing compared to the others in the series so I won't recommend it unless you're a completist. Any of the ones in the series can be read standalone, but the later ones do occasionally refer to plot events in earlier books, so there will be jokes you don't get if you haven't read them in sequence.
He's even done a few collaborations recently with Arthur C. Clarke, which worked out quite well I thought. Two of them are a dualogy called the "Time Odyssey," but each novel is essentially standalone without much impact on the other. Those two are "Time's Eye" (the first one) and "Sunstorm" (the second). Before doing the dualogy, they wrote a nice short novel titled "The Light of Other Days" which is one of the most inspirational books I've read in years. All three of these books are near-future and could be termed hard SF, though they don't explore the underlying science as deeply as many entries in that category do.
And I have to second the recommendation for Douglas Adams- "Hitchhiker's Guide to the Galaxy" is a phenomenally good and funny book, as are its sequels "The Restaurant at the End of the Universe," "Life, the Universe, and Everything," and "So Long and Thanks for All the Fish." There's also a fifth book in the series, "Mostly Harmless," but personally I found that one a bit nihilistic and depressing compared to the others in the series so I won't recommend it unless you're a completist. Any of the ones in the series can be read standalone, but the later ones do occasionally refer to plot events in earlier books, so there will be jokes you don't get if you haven't read them in sequence.