This is one of the great questions of the computer book publishing business. Once upon a time, many “experts” felt that the installed base for a piece of software had to exceed 100K before it was worthy of a book. Why 100K? The thinking was that you’d try to capture a minimum of 10% of the market with a book, which means you’d sell about 10K copies. Let me tell you something: Capturing 10% of the prospective market with a book would be pretty phenomenal. It may have happened a time or two, but that’s a very high success rate.
So how do you judge when it’s time to roll the dice on a new topic? A lot of this has to do with how hot the topic appears to be today, how much it's projected to grow in the future, how soon you can get a book out the door, what competitive books are likely to beat you to market, etc. For my money, I'd rather be the first out on a small market topic with promise than the umpteenth book out on a topic with a huge base.
In general though, it's hard to get excited about a topic with only tens of thousands of prospective readers, unless the growth projections are extremely high or you can point to some other reason why a much higher percentage of prospective customers are likely to buy a book. That's still a bit of a tough sale.
One other consideration: These days, it seems like the hot new topics/books tend to be more of a flash in the pan than they were 5 or 10 years ago. The high sales rates for a hot new title/topic seems to only last for weeks instead of months. There also seem to be fewer hot new topics each year than there have been in the past. We’re all a bit starved for “the next killer app”, I suppose. Also, competitors pounce on new topics faster, causing there to be more than one book available, but I think even after factoring that in, the sales peak is much shorter these days.
At the end of the day, it once again shows that this business is more of an art than a science. You have to go with your instincts. If the topic has significant momentum and the author has a strong platform, it’s probably a risk worth taking.