First of all, authors who only have one book are disqualified. So long, Margaret Mitchell and Harper Lee.
Authors that definitely do qualify include:
Ken Kesey–One Flew Over the Cuckoo’s Nest. 
Betty Smith–A Tree Grows in Brooklyn.
I’d also include Ira Levin’s “A Kiss Before Dying.” Without Satan’s baby, Hitler clones or robot wives, Levin’s first book is a mystery masterpiece.
             
            
               
               
               
            
            
           
          
            
            
              From what I hear, Mary Shelley definitely qualifies.
             
            
               
               
               
            
            
           
          
            
            
              Thomas Pynchon-Gravitys Rainbow
             
            
               
               
               
            
            
           
          
            
            
              I’d probably say Tom Clancy counts - Hunt for Red October (and Red Storm Rising) is far better off for having seen an editor.
             
            
               
               
               
            
            
           
          
            
            
              I was going to say Salman Rushdie with Midnight’s Children , but it turns out he had one called Grimus  that came out six years earlier. So I’ll just second Joseph Heller.
             
            
               
               
               
            
            
           
          
            
            
              **Harper Lee **- To Kill a Mockingbird; first and only
**Margaret Mitchell **- Gone with the Wind
Wasn’t Peyton Place **Grace Metalious’ **first and only book?
             
            
               
               
               
            
            
           
          
            
            
              
Both V  (1963) and The Crying of Lot 49  (1966) came out before Gravity’s Rainbow .
             
            
               
               
               
            
            
           
          
            
            
              Please read the OP. I want authors who:
Published more than one  book. 
First  book was their best  book.
ETA: Thanks, Munch  for clarifying my obviously very obscurely written and extremely hard to understand and interpret OP.
             
            
               
               
               
            
            
           
          
            
            
              Donna Tartt, The Secret History .
             
            
               
               
               
            
            
           
          
            
            
              Norman Maclean **A River Runs Through It ** published when he was 74. Young Men and Fire  published after his death is about 1% inferior.
             
            
               
               
               
            
            
           
          
            
            
              Stephen R Boyett.  Yes, he only wrote two, but he still qualifies… 
Also John Steakley.
             
            
               
               
               
            
            
           
          
            
              
                phungi  
                
               
              
                  
                    March 9, 2009,  3:00pm
                   
                   
              14 
               
             
            
              many would say that Catcher in the Rye  is J.D. Salinger’s best, although I think it pales in comparison to his other books …
             
            
               
               
               
            
            
           
          
            
            
              Walt Whitman - Leaves of Grass 
Flann O’Brien - At Swim-Two-Birds 
Charles Dickens - The Pickwick Papers
             
            
               
               
               
            
            
           
          
            
            
              I think John Grisham’s best book is A Time To Kill .
             
            
               
               
               
            
            
           
          
            
            
              
 Annie-Xmas:
 
Please read the OP. I want authors who:
Published more than one  book. 
First  book was their best  book.
ETA: Thanks, Munch  for clarifying my obviously very obscurely written and extremely hard to understand and interpret OP.
 
 
They were published after her death, but there are a couple of Margaret Mitchell books besides GWTW.
Lost Layson (sp?) is the only one I can remember offhand.
             
            
               
               
               
            
            
           
          
            
            
              
 Munch:
 
From the OP:
 
 
Darn it :smack:  obviously I am guilty of drive-by posting.  Sorry 'bout that.
             
            
               
               
               
            
            
           
          
            
            
              Night, I disagree. O’Brien’s The Poor Mouth is a comic masterpiece and a powerful glimpse of racial discrimination to boot.
             
            
               
               
               
            
            
           
          
            
            
              I always thought Jacqueline Susann’s “Every Night Josephine” was her best book.
Of course, her others made lots more money.