I also went into the series starting with Ys Book I & II. A legend that will always shine in its glory.
After Ys I and II, 4 sequels and one prequel were made.
You will find Ys III: Wanderers from Ys for many gaming consoles and old japanese computers (MSX2, Genesis, Super Nintendo, Turbo Grafx CD, PS2, NES, etc.) Note though that many of these are available only in japanese, and some has a few english patches. Ys: Oath in Felghana in a quite recent remake of Ys III for PC. It has an english patch, which you can adquire by either donating $5 to the patch creator (NightWolve) or by downloading it somewhere in the web, which wouldn't be very nice. I suggest to donate him something if the patch interests you. I, myself, will donate him 5 bucks sometime. He deserves it, and much much more...
Ys IV had 3 version: SNES, TCD (Turbo Grafx CD) and PS2. None were officially released in english, but the SNES and TCD ones have a patch. There are some patches for the SNES', but there's only one for the TCD version, which you can download in
NightWolve's blog, the patch creator.
Note though that all the Ys IVs are totally different from each other. Names:
SNES' ---> Ys IV: Mask of the Sun
TCD's ---> Ys IV: The Dawn of Ys
PS2's ---> Ys IV: Mask of the Sun - a new theory -
Ys V was only released for the SNES and recently to the PS2, only in Japan. (Note: the SNES version is a lot better). Unfortunately there's still no english patch, though one may be done sometime, maybe next year,
hopefully.
Also note that there are 2 versions for the SNES: the normal one and Ys V Expert, which has a slightly increased difficulty and has a secret Time Attack mode.
Ys VI was initially released for the PC only in japanese, but Konami ported the game to the PS2 and PSP, and released both version in english as well.
It's called Ys VI: The Ark of Napishtim, though the PS2 and PSP versions have dropped the "VI" from the title for some reason.
And then there's Ys ORIGIN, a game released only for PC in Japan. Currently, there are no patches. Don't get your hopes up, but an english patch may come someday.
And that's the basic stuff about the series. xD
Also note that each game is INCREDIBLE, each one in their own way.
To finish this, let me tell you the PC version of Ys I and II.
Ys ETERNAL ------> remake from the original Ys I. It was released in 1998. It DOES NOT have an english patch.
Ys II ETERNAL ---> remake from the original Ys II. Not sure when it was released, but I think it was an year after Ys ETERNAL. It also DOES NOT have an english patch.
Ys I.II COMPLETE ---> this one obviously contains Ys I and II. This Ys I version is a slightly improved version of Ys ETERNAL. But the Ys II within is identical to Ys II ETERNAL. This one HAS an english patch, which you can also download in
NightWolve's blog, since he created that patch as well. Also just so you know, this game was released in two separated CDs or in one single DVD. And this game is NOT called "Ys I.II ETERNAL COMPLETE". That's just a very bad..."typo" that you will find frequently.
I think that's it. Sorry for the looooooong post. Hope that I could provide you some help. xD