did he write the libretti?

Yes - it was VERY important to him that he built the whole shebang himself. The story is his own redaction of the mediaeval sources. Look up "Gesamtkunstwerk".

He was probably the best own-libretto writer among all opera composers. Look at Michael Tippett or Harry Partch for some thoroughly cringeworthy moments in the text. The music covers these but at times you'd rather it was being sung in Uzbek. Wagner never sounds silly.