"You will, I am sure, agree with me that . . . if page 534 only finds us in the second chapter, the length of the first one must have been really intolerable."--Sir Arthur Conan Doyle

This is a genre-specific question, and can only be answered in approximations.  And a good book is always better than these measures of it, making length requirements and other such considerations meaningless or irrelevant.  Still, the average is a good thing to know.

a. In poetry, a manuscript is often described as being an absolute bare minimum of 48 pages, with 48 to 64 pages often being the requirement for contests.  In recent years, however, longer books have been published regularly, though still usually under 100 pages.  These are published pages, which--depending on such design considerations as font and physical book size--are usually a little smaller than a typescript manuscript in Courier font, 12 point.  A published page approximates, and is often in--Times New Roman font, 12 point.

b. In fiction generally, about 50,000 words is a decent minimum.  This would be about 300 typescript pages in Courier font, 12 point.  In fiction volumes, because of the full page text, the book pages will be considerably fewer, rather than more as in poetry.