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:
Common mass market paperbacks are generally 250-300 words per page. Trade paperbacks typically run a similar count, due to larger type and wider margins. Hardbacks and trade paperbacks are printed from the same plates, most times, so word counts will be the same.
That said, this is variable. Occasionally, a publisher will reduce type size by a point to cut nearly a hundred pages from a very long novel, and this might push the word count on a page to as much as 350; other times, they'll bump type size up a point to "fill out" a short work, dropping the word count per page as low as 200 words. Neither of these practices are as common in this day of electronic typesetting as they were, say, in the 1970s, in part because there are other ways to accomplish the same ends, and in part because the physical thickness of a book on the shelf at the bookstore matters less than it did (in large part because most books sold today aren't on paper at all).