It really depends on the type of book you are trying to write. You wouldn't want to make a children's book 300 pages, but at the same time, you wouldn't want to make a book that discusses WWII to be 10 pages.
That being said, I think that no matter what you are writing, it's important to be concise or to have tight prose so that you don't bore your readers as well as to impart useful information or make it entertaining to the reader (again, not sure what you are writing).
I would say you're probably looking to write around 150 pages - 250 pages, though there are many more factors other than page count that should be taken into consideration.




