This is often asked by bookstore owners or architects who are designing a bookshop. I have explained this matter several times. But to make it clear again I will say that the space or distance between bookshelves ranges from 1.5 meters for small bookstore to 2.5 meters for large bookstore. In my bookstore I choose the smallest one which is 1.5 m because I don't have much space left after the whole floor area has been filled with bookshelves, tables, chair and books.
Please, provide larger space, at least two meters at the entrance and exit doors so that your customers won't experience bottle neck. You know what I mean? Fill an empty bottle with water and turn it to pour it out again. See what happens? The water will slow along the small tunnel leading to the mouth of the bottle. So, if the entrance and exit space doors in your bookstore have small spaces, your customers will not be comfortable with it. If your bookstore has only one door, you need to ensure that there are no objects in front of it that could blockade the flow of the customers moving in and out of the bookshop. The first bookshelves that they see should be the newly arrived books. It does not always have to be a bookshelf. It can be a table where new books are displayed on it.
Also read: Standard Space Between Bookshelves