origin: French, bouquin= old book + iste= -ist (doer, maker, procurer)
Technically, it's a French word for a seller of used books.
However, that doesn't cover the full charm of these outdoor, makeshift -- or as we say now "pop up" -- retail establishments dotting the Seine, often with old men in rickety chairs puffing away on a pipe, bundled in a coat, protecting and representing their wares. The bouquinistes look much today as they did over 100 years ago, currently favored by the government, but once the scourge of "brick and mortar" or established booksellers paying far more in overhead.
Once upon a time, you could line up most anything along the bank, stuffed with books and merchandise -- suitcases, cigar boxes, old hats -- but now built-in "boxes" (or kiosks) lock up at night for convenience.
And so to quote Wikipedia's quote, because it's that good, it is said the Seine is "the only river in the world that runs between two bookshelves". You may also find magazines, posters, stamps and sundry souvenirs or antiques by those being even craftier with their bit of rented & historic space. Neither charm, nor printed paper, is quite yet dead even in our digital and ever-more electric world.