At the moment, I'm teaching a subject that is all about writing chapter books. In Australia, that is. Chapter book is a tricky term. In the USA, it can mean any book for children that has chapters. However, in Australia it generally means those books published for kids who are emerging or newly independent readers. In other words, they're what you read when you've had enough of school readers (the John and Betty ones, although they're vastly improved these days), but you're not quite ready for novels.
Novels generally start at around 15,000 words. My students are struggling still with exactly what makes a chapter book, so I've set the parameters to make it easier (even though I know there are plenty of chapter books outside this boundary). We're looking at anything from 1,000 words to 8,000 words, with illustrations. For those of you who like to write whatever you want, it might seem a tad restrictive to say a book must have 1500 words, but this is the recommended number for an Aussie Nibble. Other series, particularly those put out by educational publishers, are even more restrictive. They will set a word count, a range of topics, a target readership and the number of pages this all works out to be.
Like it or not, the word count issue applies to nearly all books. Sometimes it depends on what you're writing and who you are. If you're a new fantasy writer, you'll be told that more than 120,000 words is frowned upon, and around 100,000 is your best bet. Too bad if you've written a 300,000 word epic. (However, if you're well-published, the word count doesn't really apply any longer.) If you write for young adults, you're looking at around 40-50,000 words. Category romances have word limits. Even literary novels are unlikely to be much outside the 70,000-80,000 word count. Of course, you can write whatever you want. But these days you'd do well to have a fair idea of what the average word count is in your genre/form, and have a darned good reason for going outside it.
The pesky problem arises in your query letter. You can't lie (well, you can, but when you get caught out you're going to look unprofessional). So an editor or agent to whom you're pitching, say, a middle grade fantasy is going to feel a fair bit of dread when you say your novel is 90,000 words. (Never mind Harry Potter - I've tried that arguement and it doesn't work!) And if you're pitching a literary novel of 38,000 words, the same suspicion will arise, regardless of The Bridges of Madison County.
I'm pondering all of this word count stuff because I'm currently trying to write texts for very new readers. Texts that have a maximum of 50 words but must still tell a story. Other texts that have a word count of 350-400 words but must still tell a darned good story, with a beginning, middle and end. It's practise that helps, I find. You write one, and keep it as tight as you can, then you are 100 words short so you have to fill it out with more exciting bits. Or you are 100 words over, and you have to cut out every single fluffy extra phrase you can find.
It's actually really good for your writing to do this. I remember one year I only had one suitable story for the Age Short Story competition, but it was 480 words over the 3000 word limit. It took me two days, but I finally got it down to 2998 words. I learned a lot during that exercise, and I've never gone back and added the words back in again (after I didn't win). When I read the story a few weeks later, I realised that the cutting had improved the story immensely. An even better lesson.