“Practically everybody in New York has half a mind to write a book, and does” is a witty line that was credited to editor Bill Targ (1907-1999)  in 1946. “The trouble with the publishing business is that too many people who have half a mind to write a book do so”—a similar quote, but without “New York”—has been credited to William Targ since at least the 1970s.
 
American comedian and actor Groucho Marx (1890-1977) has been credited with the “New York” line since at least the late 1960s, but Marx was well-known and it’s unlikely that the earlier printed citations wouldn’t have credited him.
 
The “half a mind” expression was popular in the 19th century. “I’ve half a mind to write a book” was cited in print in 1855. “I’ve half a mind to write a magazine poem” was cited in New York (NY) Press 1894, along with the punch line, “Half a mind seems to be about enough for that sort of thing.”
 
[This entry was assisted with research from the Quote Investigator.]
   
 
Wikipedia: William Targ
William Targ (1907 – July 22, 1999) was an American book editor, well respected in the field of commercial publishing. He is known for publishing Mario Puzo’s novel The Godfather while editor in chief of G. P. Putnam’s Sons.
 
18 January 1855, Daily Evening Traveller (Boston, MA), “Poetry: ‘Brass’ from the Portsmouth Journal, pg. 4, col. 1:
I’ve half a mind to write a book
On Brass!—it would be rare.
 
21 January 1894, The Daily News (Denver, CO), “Quips of the Day,” pg. 12, col. 7:
“I’ve half a mind to write a magazine poem.” “All right. Half a mind seems to be about enough for that sort of thing.”—New York Press.
 
28 November 1946, Wisconsin State Journal (Madison, WI), “Notes for You,” pg. 23, col. 6:
The same magazine (Counter Points—ed.) quotes Bill Targ: “Practically everybody in New York has half a mind to write a book, and does.”
         
3 August 1947, Lubbock (TX) Avalanche-Journal, “The Woman’s Angle” by Margaret Turner, sec. 2, pg. 1, col. 1:
It seems to us these days that practically everybody has half a mind to write a book, and does.
 
20 July 1951, State-Times (Baton Rouge, LA), “Pull Up a Chair” by Neal O’Hara, pg. 4, col. 6:
Observes the Cleveland News: “The trouble with publishing is that too many people who have half a mind to write a book do so.”
     
22 September 1974, Sun-Standard (Blue Island, IL) “Sparks of Wisdom,” pg. 4, col. 5:
The trouble with the publishing business is that too many people who have half a mind to write a book do so. (William Targ)
 
New York (NY) Times
‘No Author Is a Man of Genius to His Publisher’
By WILLIAM ROSSA COLE; William Rossa Cole is a writer of children’s books and the editor of 40 anthologies.
Published: September 3, 1989
(...)
The trouble with the publishing business is that too many people who have half a mind to write a book do so.
William Targ, editor
 
Google Books
20,000 Quips & Quotes
By Evan Esar
New York, NY: Barnes & Noble Books
1995, ©1968
Pg. 51:
Practically everybody in New York has half a mind to write a book — and does.—Groucho Marx
 
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Groucho Marx: Practically everybody in New York has half a mind to write a book, and does. | #writingtips #writing
10:43 PM - 15 Aug 2015