Last Wish and Sword of Destiny are collections of short stories released in that order. After they turned out to be successful the author released a proper series with one story line consisting of five books: Blood of Elves, Time of Contempt, Baptism of Fire, The Swallow’s Tower, Lady of the Lake (oh and there's also Season of Storms which is technically a prequel released long after the main series conclusion). I've never read them in English, but from what I've heard the translations aren't amazing (+ Lady of the Lake is yet to be translated, it's set to come out sometime in 2017), which is a damn shame because to my taste (and this is of course entirely subjective) The Witcher series is THE quintessential fantasy book series. I was amazed by it when I first read it in high school and it was even better than I remembered it when I decided to re-read it a decade later. It's obviously entirely subjective, but while it's slightly smaller in scope than The Song of Ice and Fire it still gives you this amazing huge world full or intrigue without overblowing it and instead focusing on what's really important - well-written characters. While the short stories aren't all great the main series doesn't skip a beat and it's so enjoyable to read through and through that at least once a month I find myself tempted to re-read it again.
EDIT: After small research it turned out that people have gripes with the community translations. The official translations (while not perfect) are reportedly a lot better. With that in mind, if you enjoy fantasy do yourself a favor and read the series
@CycleVancouver. The proper read order would be to go with The Last Wish and Sword of Destiny first and then the series (ignoring Seasons of Strom till it get's officialy translated, also even though it technically is a prequel there's this one thing in it that makes a lot more sense if you read it after the series).