The pros and cons of getting an advance for a book you haven't written yet

Justine Larbalestier posted recently about why she's chosen to write her new book before selling it rather than selling the book on the basis of a proposal, and it led to some interesting discussion in the comments. Here's one of the best reasons not to sell your book first (it's Maev Kennedy writing in the Guardian):

According to new research by a leading expert on the dictionary, rather than working slowly but steadily with his assistants on the dictionary for the full nine years, Dr Johnson became completely bogged down in the work, realised he would miss his deadline, and simply abandoned the job, ignoring increasingly frantic messages from his commissioning editors.

However, Dr Anne McDermott, a senior lecturer in English at the University of Birmingham, who has spent years studying all the surviving original manuscripts and sources, has a different explanation. She will announce tonight, in a lecture at Dr Johnson's House museum in London, that Johnson was in fact paralysed over the work for years, to the despair of his publishers by whom he had already been paid a hefty advance. In the end, she contends, he finished the dictionary in just over two years with only two assistants.

...

She believes that only a threat to break into his house and seize the manuscript - which the publishers mistakenly thought was almost finished - which got Johnson back to work. He was bribed with a guinea for every page delivered to the printers, and although this time he could afford only two assistants, they raced through the work and finished in two-and-a-half years.

I wish I could have been there to hear the lecture, but perhaps it will be printed somewhere soon--the TLS occasionally does pieces like that. (Aside from her many other accomplishments, McDermott is responsible for the wonderful CD-ROM version of Johnson's Dictionary of the English Language that I use in some of my classes--but I am hoping that they are going to release an online version sometime, the interface of this one is tricky to use and at least at Columbia they never have got it set up on a LAN.)