Chuck Palahniuk - all of his books I like to have on hand at all times, but Invisible Monsters especially.
I also have a slight obbsession with collecting the Christopher Pike books - yes, I know they’re teen thriller novels, and I also know they mostly suck, but he’s great. I even got the 'rents hooked on the Last Vampire series. 
Tolkien
Robert B. Parker
Michener
Terry McMillan
C.S.Lewis
Grisham.
By all means! Yes. There are several authors whose books I just have to own even after reading them and a couple I have reread so often that the books wore out and I had to order in replacements.
I LOVE the Talisman and Insomnia - my favorite two King novels (and I’ve read them all, even the new one, Dreamcatcher.)
I also own a beautiful leather-bound collection of the Chronicles of Narnia. I begged and pleaded and hinted every day for months leading up to Christmas, and was justly rewarded. 
George Macdonald Fraser’s “Flashman” series, although I’ve read and enjoyed his WWII stories.
Patrick O’Brian, in book and in tape format.
Rex Stout’s Nero Wolfe series, although I have so many I haven’t felt the urge to complete the set.
A lot of Stephen King, although I agree that I will never be able to catch up until he’s been dead, oh, about a decade. (I remember fondly buying each “Green Mile” book as it came out. That was neat.)
“Green Mile” reminds me that I’d buy Scylla’s work if he ever gets around to putting it between covers. That sheep story’s got me on edge.
Oh, and Mark Twain. I’ve got the Running Press two-volume edition (about 2,000 pages), a smattering of Library of America works, the slipcased reproduction of Tom Sawyer, first editions of “Innocents Abroad” and “Across the Equator,” and I’m seriously considering the autographed edition put out by Oxford University Press (autographed by those who wrote the foreward and afterwards to each book, including people like Vonnegut, Toni Morrison, Roy Blount, Arthur Miller, Gore Vidal.) Only $500.
Oh, and Hemingway. Just got the first of A.E. Hotchner’s “Papa Hemingway” in dust jacket for .75 at a book sale. Finest kind.
I own hundreds of books, but there are few authors whose complete works I possess, besides Shakespeare. I own all of David Eddings fantasy books (but not his two other books- High Hunt or the Losers, though I’ve read the later), I own all of Douglas Adams books (except “Last chance to see” which was depressing and non-fiction) I own all of the Adrian Mole books by Sue Townsend, I have all of the “orginal” Vampire books by Anne Rice (the first five) and all of the books in The Kent Family Chronicles by John Jakes, and that’s all I can think of off the top of my head though I know there are more. Other than that there are authors whom I own “most” of their books, but not all in even a series. At the moment I’m working on getting the rest of Sharon Kay Penman’s books.
elfkin477, you said:
you should read Merrick; it continues the storyline of the “original” 5.
Margaret Atwood and Jennifer Crusie. I would own all the available Harry Potter titles, but I’m waiting for them to finish and come out in a special hardback edition.
Yes. I am a Frithjof Schuon completist. I have only one more book of his to get, and that is a big, expensive art book of his paintings.
Let me put it this way: I went out of my way to get two German books of his that had never been translated into English. Poetry books (he wrote his prose in French and his poetry in German). I learned German just so I could read those poems.
Richard Russo and Kent Haruf.
Brian Jacques. I am still missing lots though, cause I am a poor college student, and some that I lent out have not come back 
Philip K. Dick – still missing some of his earlier out of print works that have not been reprinted, (like Counter-Clock World)