A quote from John Gregory Dunne by way of a Jonathan Yardley essay about Dunne (the whole thing is worth reading):
"Because one has written other books does not mean the next becomes any easier. Each book in fact is a tabula rasa; from book to book I seem to forget how to get characters in and out of rooms -- a far more difficult task than the non-writer might think. Still I went to my office every day. That is the difference between the professional and the amateur. The professional guts a book through this period, in full knowledge that what he is doing is not very good. Not to work is to exhibit a failure of nerve, and a failure of nerve is the best definition I know for writer's block."
(BookWorld is actually stuffed with good pieces this week -- Dirda on Conan, John Crowley on a new bio of Robert Louis Stephenson, etc. Plus, there's the Style pageant piece.)