Is it permissible for the owner of a bookstore to refuse to increase the share of the employees who participate with him in the profits, on the grounds that he is the owner of the bookstore, even though they exert great effort and bring fame to the bookstore, while he does not exert any effort in the work entrusted to them? And is requesting an increase in the share considered a sin?

The contract between you and the bookstore owner is a lease contract, and in contracts, what matters is their meanings, not their wordings. The rent for the lease must be known, as the Prophet (peace be upon him) forbade hiring a worker until his wage is made clear to him. The majority of scholars prohibit the rent from being a percentage of the machine's income due to the uncertainty of the wage, while some scholars permit it. You do not have the right to compel the lessee to add another person to work with you, and what you did by involving a third person without the bookstore owner's consent is to be borne from your own rent.