How are you thinking of "pen?" A writing instrument with a self-contained reservoir for ink so that you don't have to dip? The first pens were dip pens, which worked on the same principles as the quill and ink pot. They are a very old idea. You dip it in the ink and write until you run out of ink.

If, as I'm guessing, you're thinking of fountain pens, there were many methods of containing ink in the body of the pen invented through the ages, but the inventor of the first modern fountain pen is usually given as Lewis Waterman, in 1883. Their company website makes a big to-do about it. Prior to that most fountain pens were fussy and spitty and not very popular. His was a commercial success and brands began duking it out over fountain pen designs. The fountain pen and the dip pen co-existed for a while though. So maybe around 1900-1910 for when the self-contained pen took over.

Check out this neat site on US president's pens. Sort of an interesting snapshot of the evolution of writing instruments.

There's not a lot of books on fountain pens actually, but this guy is the go-to Internet expert for fountain pen collecting.

The Story of the Invention of Steel Pens, With a Description of the Manufacturing Processes by Which They Are Produced, a nice free ebook originally published in 1890.

A brief synopsis, I don't remember the names, but one enterprising gentleman's silversmithing business was declining, entrepreneurially he was working on replicating the quill pen in a more robust form, made a sale, which ballooned quickly. Of course, others started copying him, offering the unheard of "a gross of pens" for 1p, or some such low price.

The ebook is a good read as it is approximately contemporaneous, and documents a seemingly insignificant yet class-busting industrial advancement: cheap pens for everyone. Goose quills and the artisan level skill to turn them into a short lived pen made them expensive and not an item for the lower, illiterate class.