Silvia, Paul J., How to Write a Lot: a Practical Guide to Productive Academic Writing (Washington : American Psychological Association, 2007) ISBN: 9781591477433
This slim book is not just good for game designers and fiction writers. It can be used by GMs, by players involved in writing up PCs (or anything else), by anyone who is involved in creating for their campaign, and so on. Simply, it presents arguments for basic, practical ways of increasing productivity. An example is the recommendation to set aside a regular amount of time to write in, and only to write it. To schedule, that is, time to create. In gaming terms, that means we should set aside a regular amount of time in which to dedicate our efforts at creating, whether it be a gaming supplement, and adventure, or a campaign world, and so on.
There are other, and more tips (and I won't go into these) but the end result is more material created more regularly. If you do NaNo I'm sure you know the drill: writing regularly, over time your material will build up.
If there is, though, anything that I would add, it is the exhortation that, when dealing with others, game publishers, magazines, and so on, to remain professional in all of your dealings.
Good for: game designers, GMs, creative gamers