I think the idea of convergent fractions is older then Euler.
A similar idea, look up Zeno's Paradox,
http://en.wikipedia.org/wiki/Zeno's_paradoxes
Read the Achilles and the Tortoise, basically it puts the fastest man on earth against a tortoise with a small head start. Achilles never passes because first he must get half way to the turtle, then from that point another half, then from that point another half way to the turtle, and so on. Basically this idea stumped them because they could not determine after how many seconds Achilles (the fastest man on earth) would pass the tortoise. Because it seemed he would only be getting closer.
It was because at this point there was still no concept of 0 (or infinity. Zero and infinity have always been dual objects, it was always taken that you could not have one without the other.) As far as I know as the first example of an infinite converging series of fractions.
Although this is not a continued fraction, it has the same idea and I think is something a little more profound, it is I think the first example of an infinite convergent sequence, I think that questions like this got people curious. And thinking about continuous motion and things going off to infinity and such.
A basic number theory book may have more information on this. I would think a good introduction to analysis may have a bit more on convergent sequences then an algebra book would. However, no books come to my mind, it's not something I have read about; most of my books are more general and abstract this is more specific.
Wikipedia is always a good source (and contains other good sources!) I just scanned it and I think maybe I'll read it later when I have more time but that would probably be a good start. Also look at the sources for the article as often those books are pretty good too.
Actually the first recorded use of continued fractions is 300 BC by Euclid. (see wiki article)
However Zeno was around 400 BC, so the example I gave (while not a continued fraction) is older.
http://en.wikipedia.org/wiki/Continued_fraction
Good luck!