Cassandra Clare's website today. She has a bunch of links on her bio page that offer advice to writers. Here's a link she provided that I found useful. It discusses word counts when writing a book. The information was found on Colleen Lindsay's blog (for the full article, click her name).
middle grade fiction = Anywhere from about 25k to 40k, with the average at about 35k
YA fiction = For mainstream YA, anywhere from about 45k to 80k; paranormal YA or YA fantasy can occasionally run as high as 125k. The second or third in a particularly bestselling series can go even higher. But it shouldn't be word count for the sake of word count; the word count must actually be what works best for the story.
urban fantasy / paranormal romance = Usually around the 100k mark; some bestselling urban fantasy writers are able to turn in even higher word counts, but as a debut author, stick to the appropriate range.
mysteries and crime fiction = Cozies tend to be shorter than the average, somewhere around the 60k to 70k mark; most other books that fall into this category fall right around the 90k to 100k mark.
mainstream/commercial fiction/thrillers = Depending upon the kind of fiction, this can vary: chick lit runs anywhere from 80k word to 100k words; literary fiction can run as high as 120k but lately there's been a trend toward more spare and elegant literary novels as short as 65k; thrillers also run in somewhere around the 100k to 120k mark; historical fiction can run as high as 160k words or more (and again, these are just rough guides - there are always exceptions). Anything under 50k is usually considered a novella, which isn't something agents or editors ever want to see unless the editor has commissioned a short story collection.
science fiction & fantasy = Here's where most writers seem to have problems: most editors I've spoken to recently at major SF/F houses want books that fall into the higher end of the adult fiction you see above; a few of them told me that 100k words is the ideal manuscript size for good space opera or fantasy. For a truly spectacular epic fantasy, editors will consider manuscripts over 120k but it would have to be something extraordinary. I know at least one editor I know likes his fantasy big and fat and around 180k. But he doesn't buy a lot at that size; it has to be astounding. (Read: Doesn't need much editing.) And regardless of the size, an editor will expect the author to to be able to pare it down even further before publication.