The least expensive coating is generally press varnish (aka flood varnish). It’s a very inexpensive coating applied in-line or on a stand-alone press. It’s similar to press ink without any pigment but has additives that offer gloss or matte finishes and varying degrees of protection. Like ink, it can print from a plate (aka spot varnish), so a glossy varnish can contrast a matte varnish on the same surface for a subtle design element.
The bad news is that varnish offers minimal protection: even books in non-retail settings can be damaged by excessive handling on the shelf. Shipping books with varnished covers in cartons is usually a disaster. The books easily scuff the ink off against each other in transit, and your first reaction will be “What are all these white spots on my covers?” Those spots are where ink used to be before a truck bumped down the road, allowing the books to rub against each other.