When you decide to start, collect mail adresses of interested readers. Don't just assume people will stumble upon your book, seek out readers long before the book is finished.

I recently told this on Twitter to an Academic who had just finished writing a book on Cryptography in middle-age Venice (how cool is that?), but with publication still many months off.

She had never thought about collecting people's mail addresses, even though dozens (if not hundreds) of people replied on Twitter that they'd be interested in reading the book.