I'm writing something and the font is Alegreya in 13 font size. So far I have 891 words and 4 pages. How many pages would that be in a book?
There is no single answer that can be given for this, since books can be typeset in a variety of different ways, and different font faces and sizes will produce a varying number of words per page.
However, a generally accepted formatting for manuscripts is double-spaced, 12-point Times New Roman, with one-inch page margins. Based on that, it's assumed that one page will be approximately 250 words.
Here are some online "calculators" I found that will let you adjust the font face, size, and number of words in order to produce a page count estimate: