Were there any "textbooks" during medieval times? For example, books for students of medicine, sciences, or swordfighting and weaponry?

For waeponry and swordfighting, like many "arts" (meaning technics and industry), there was no real text book until the invention of printing works, because it was expensive and difficult to copy books, and on such mater, a master was suffisiant to learn everything there was to learn. So no such thing in those domain until the 15th century more or less. That is, for the exception of some manuals on governement and the art of war from antiquity, and it was not wild spread in Europe.

As for "textbook", as a corpus of text used by student, there was some. Education (beside basic writing and calculus taught to the merchant son, mostly at home and with practice) was structure by to main curriculum, that taugh the seven liberal arts: the Trivium (grammar, logic and rethoric), which was the lower level of education, and the Quadrivium (Arithmetic, geometry, music and astronomy), which was the higher level of education. Both the Trivium and Quadrivium were based on Aristotle's definition of sciences, and so were taugh mostly through is work, as well as those of Pythagoras, Plato, Porphyre, and various antic philosophers, along with their Roman successors. It's hard to give title for the books, as many of them are lost, but Aristotl "Physics", "On the Soul", "Sophistical Refutation", "Rhetorics" and "Metaphysics" were among the books that every student would encounter in his study, along with transcription of Pytagoras works by Heraclite "On nature" and "On politics". For the specific case of medicine, the complete work of Galenus and later of Avicenna was the basics of all medecine until the 17th century. And of course, the Bible was the first book that was used to taugh reading and writing, even long after the medieval times.