Booksellers are as just about as old as the written word. For example in ancient Greece scribes would set up shops where they'd copy out and sell books on demand. This probably would have been incredibly expensive, though, so maybe what you mean when you refer to books being "for nobles"? However, in Rome during the late republic, Martial mentions his first book of epigrams being on sale for five denarii, which would have been about five times the daily wage of an average worker. So I suppose if an average person were literate and really really wanted to buy a book, it wouldn't be entirely out of reach. But still probably only for the wealthy.

Starting in the 15th century, printing democratized access to reading material surprisingly quickly. Cheap and ubiquitous weekly newspapers were springing up all over Europe by the early 17th century, and printed books and pamphlets would have been available long before then.

The Bertrand Bookstore in Lisbon, Portugal, has been open since 1732 and claims to be the oldest continuously operating bookshop in the world.

The early 19th century saw the emergence of the "penny dreadful", cheaply printed novels aimed at the poor. So not just "not nobles", but literally the bottom rungs of society. At this point, the major obstacle to getting cheap reading material to ordinary people was literacy rates, not the expense of books themselves.