I’m going to lay out the recommended lengths of every kind of book here. It is a question that every author has and yet, there seems to be no great resource to find such vital info. One caveat before I continue- these numbers aren’t my opinion. This is info from publishers and agents of various literature. If you want to break these rules, be my guest. If you are self-publishing, you can write your own rules. If you want a publisher to pick up your book, you need to adhere to these because the junior editors who fill wastebaskets everyday will know these rules.

How heavy should your book be?
Adult Novels: The great American novel ranges from 70 to 110 thousand words. If you shoot for 85,ooo, you should be fine. Straying too far from this range is dangerous. There are logical reasons as well as psychological ones. The longer the book, the more expensive to print and ship. But also, people might not want to start such a long book. We are very accustomed to a certain pace and story arc in our literature. A long book means there is something more than what I need in there; more description or more characters or more build up, etc. Bigger isn’t better. And shorter isn’t better either. A short novel might have to be priced lower than others because of perceptions of value. In actuality, a shorter novel might just be more concisely written. Or it may be lacking some fundamentals like character development. Oh, the doubts that can arise from a tome too slim.
Romance Novels: The staple of the literary market is the romance novel. They outsell any other genre of book. One reason is the size. Your average bodice-ripper weighs only 75,000 words. So they are consumed much faster. The very logical reason for this length is the very unlogical result they have on their readers. Women read romance novels for the intoxicating high they provide. The 70 to 80 thousand word range gets to the happily ever after in just the right amount of time. Too soon, and the anticipation isn’t there. Too late, and the reader gets frustrated. The historical romance books are geared for more intellectual women and should therefore be slightly longer. Those readers are looking for a little intellectual nourishment with their emotional rush.
Sci-Fi/Fantasy: Nerds, like myself, devour boxloads of books in this second-best-selling genre. Like the romance novel does for women, the sci-fi/fantasy novel does for men. But while female escapism books are shorter than standard novels, men’s escapism books are longer than the standard. 110k words is the target for these tomes, give or take 10k. This is because the protagonist goes through more development. In romance novels, the man is already powerful, rich, and handsome. In sci-fi/fantasy, the hero is unremarkable and, through dangerous adventures, evolves into a great hero. There is something about that story arc that appeals to the male brain. Nevertheless, it takes longer to write that story arc so the genre simply has more words.
Young Adult/Teen Novels: Normally referred to as a YA novel, this genre appeals to readers who are facing teen angst. They are not formulaic and rarely predictable, just like high school. These books tend to be quite short for novels at around 60 to 65 thousand words. You can stray from this range by about 10k in either direction and still fit within the genre. The reasoning is that these books are primarily read by teen girls. They have busy lives, homework, friends, sports, family stuff, etc. Reading is probably not a huge part of their daily schedule- and if it is they are probably already reading adult novels.
Other Novel Genres: Any genre not covered above specifically, likely falls into the 70 to 110 range with emphasis on the middle of that range. Westerns, Memoirs, Historical Fiction, Mysteries, and such are all in that range.
Middle Grade Readers: These books, also called chapter books, range from 20 to 45 thousand words. A good rule of thumb is to identify the exact age of the target reader and multiply by 3000 words. So a 7 year old will enjoy a 21,000 word book while a 12 year old will enjoy a 36,000 word book. What appeals to an 8 year old will not appeal to a 10 year old so know your audience.
Picture Books: Storybooks for little kids should not have many words. Your target should be 300 to 600 words. Quality definitely trumps quantity for picture books. If you cannot say what you want to say in those few hundred words, then maybe this is the wrong way to express yourself. Not to be overly critical but, if I have to read a thousand word manifesto at bedtime, I’m going to make sure that book vanishes from my house. Neither three-year-olds nor the parents of three-year-olds have the patience or sanity to read for more than a couple minutes.
Alright. I think that covers most every book you might write. Remember, these are just guidelines. Writing isn’t about numbers. Do yourself a favor and do the writing first. Then, when you are done, make some adjustments if need be.