The complete list of the UT books can be found
here.
If you want some advice about which book would be the best one to buy next, then I would recommend you either "Samurai" (book 2) or "Grey Shadows" (book 13), since they're the following books to the ones you have and are also among the best. Others than these 4 already mentioned, my favourites would be "Lone Goat and Kid" (book 5), "Circles" (book 6), "Daisho" (book 9), and "Seasons" (book 11); but, actually, any book would be a great choice (well, that should be obvious, shouldn't it?

).
For some reason, ever since "Grasscutter" there has been some kind of continuity between the stories. The book "Seasons" takes place in a time period of about a year or so, and every story that has been published since "Grasscutter" (and that includes 4 more books and quite a number of comic-books, which will probably take book #17 and #18, and probably #19) takes place on a time period that goes from summer of 1605 until autumn of the same year! I actually like it, since it allows some continuity between the stories (all books from #12, although they can be read by any new UY reader who hasn't read any other book, are linked one to another), and also happens that the best UY stories ever are being made right now (the "Duel at Kitanoji" story arc is awesome, and the current "Father and son" story arc is as great as the other). However, if you've read "Grasscutter" and are planning to read "Grasscutter II: journey to Atsuta Shrine" (15), then you would probably want to know what happened in the few weeks that separate one book's facts from another's (which can be found in "Grey Shadows" (13) and "Demon Mask" (14) ).