The Binding Constraint on Book Length

Nonetheless, a  non-exhaustive but still wide spatiotemporal coverage has its own epistemic and methodological problems, especially if attempted within the realistic constraints of page economy (scholarly publication being increasingly subject to the harsh rules of profit and loss).

(Wael B. Hallaq, in the introduction to his Sharia: Theory, Practice, Transformations, a book more than six hundred pages long)

Hallaq's complaint takes it for granted that the constraint on how long a book he can write is how long a book his publisher is willing to publish. I suspect that the more important constraint is how long a book his readers are willing to read. As some evidence, I note that he also published a drastically shortened version of the book, discussed in a recent post here. That version, about a third as long and about half the price, ranks 152,814 on Amazon. The longer version ranks 871,161. Amazon does not provide actual numbers for book sales, but my guess is that the latter figure represents a fraction as many as the former.

It is possible, of course, that the still longer book he would like to have written would do better. There is a straightforward way in which he could find out. CreateSpace, Amazon's print on demand subsidiary, will publish a book at negligible cost to you, put it up on Amazon and send you the resulting royalties; the rate depends on the price you choose to charge. If you are willing to accept royalties of 10%, the price for a 600 page book is about $16. So Hallaq could  publish twice the number of pages he did as a two volume work and sell it on Amazon for a total of $32, which is less than half the price of the existing book.

My guess is that it would not sell many copies, even at that price, but I could be wrong.