One book at a time... though I'm usually doing the research for others while I'm writing, but that sort of research is fairly desultory and I like to stick to the book being written - and writing a book concentrates the mind so the research is more productive.
Looking back, of course, it was irresponsible, mad, forlorn, idiotic, but if you don't take chances then you'll never have a winning hand, and I've no regrets.
Judy couldn't move to Britain for family reasons, so I had to come to the States, and the U.S. government wouldn't give me a Green Card, so I airily told her I'd write a book.