Money, money, money. The size of your book will determine how expensive your book is to print. So think carefully before writing as many pages as you possibly can as this will make it more expensive. A more expensive book means fewer copies are sold. Now this doesn't mean you should make your books 100 pages, it just means consider the cost when writing your book. If you've got the information, why not write a series rather than one big book that requires super-human strength to lift.
Your readers and the market control you. After all, you need them so you must give them what they want.
For example, currently there is a push to make large books that will require a lot of time to read, especially in Science-Fiction and Fantasy (SFF). This doesn't mean you should write big books for the sake of writing a big book. However, if your book is pulp length (70,000 words or 200 to 300 pages) you are probably going to have problems selling it.
When people find a character, or group of characters, they like, they want more, more, and surprisingly, more. This means that they want a series of books. It doesn't mean you have to write a series, but it does mean you should consider it. Especially if your book is on the long side. After all, a fan of the first book will normally buy the next books in the series. After a few they will probably try reading some of your other books. If you have an idea that you can easily stretch into multiple books making them all classics, do so. A series will sell much better than a single massive volume. And selling books is the name of the game.
Having said that, if you feel like you only have enough ideas for one book only write one, rather than a series. People will prefer a single well written book full of brilliant ideas than 3-4 books where it feels like the writer has just written more for money.
Hardcover is more expensive than paperback, there is no way around it. Because of this people have a tendency to pick up the soft cover, and it's also easier to transport. If you have a good fan base then a hardcover is a good idea for libraries and serious collectors. But otherwise you should start with paperbacks. People have to know they're spending the money well by purchasing a book they know they'll enjoy when they buy an expensive hardcover. If they don't know you're a good writer they won't be willing to spend the money - normally.
The best way to find what your book needs is to look at other books in the genre. Look at the length that is most common for the top books, and use that as a baseline of where you want to aim for. There are normally 250 words a page, and for SFF books there're either 400 or 700 pages, roughly. This amounts to 100,000-125,000 and 175,000-200,000 words respectively.
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Glen Ford is an accomplished consultant, trainer and writer. He has far too many years experience as a trainer and facilitator to willingly admit.