The entire Ogre Battle series is based on a novel (or perhaps just a background history) that creator Yasumi Matsuno wrote. Episode V is Ogre Battle: March of the Black Queen. Episode VI is Ogre Battle 64: Person of Lordly Caliber. Episode VII is Tactics Ogre: Let Us Cling Together. The GameBoy Advance's Tactics Ogre: The Knight of Lodis is a prequel to Episode VII. The Neo Geo Pocket's Ogre Battle game takes place at the same time as the original Ogre Battle.
Here's an informative GameFAQs post (a rare thing indeed) where someone translated a 1995 interview with Matsuno.
Episodes I through IV were never made, nor was episode VIII. And they will likely never be made now that Matsuno's at Level-5 and the rights to the Ogre Battle/Tactics Ogre series lie with Square Enix.