What is the antonym of "rarissima" and "rariora"?

Solution 1:

I suggest that the nearest Latin antonyms would be trivia or vulgaria.

EDIT If you don't mind having English then 'commonplace' should serve.

"You're like to find rarissima/rariora in an antiquarian bookstore, but the average retail bookstore like Barnes & Noble will only stock the commonplace."

Solution 2:

Of course, the common phrase for the antonym is mass-market books.

But purely as a single word, a possibility is potboilers:

[Merriam-Webster]

: a usually inferior work (as of art or literature) produced chiefly for profit

// The problem with Oscar Wilde's 1895 potboiler An Ideal Husband is precisely the thing for which its author is routinely praised: its flood of exquisite witticisms.
— Justin Hayford, Chicago Reader, "A new production of An Ideal Husband humanizes Oscar Wilde," 13 Apr. 2018

Although there is some judgment behind this word (but that may also work in favour of a complete antonym, if rare books are associated with the intelligentsia), it has the implication that these are books that are popular, common, and sell. You can find them everywhere. They're the kind of books that you'd find in an airport or train station store, and which quickly grab people's attention as "easy reads" or "pulp fiction."

A synonym for of potboiler, one which doesn't necessarily imply low quality, is page-turner. But while many common and popular books likely are page-turners, the reverse isn't always true.

Solution 3:

How about

ubiquitaries

The meaning of this word is not restricted to books, but in your sentence it would be clear it is referring to books. From the OED:

ubiquitary, n. and adj.

A person or thing that is, can be, or seems to be, everywhere at once; someone or something that is ubiquitous (in various senses).

For some reason it is 'frequently used (chiefly humorously) of insects'.