Brian Jacques's Redwall stories contain a huge number of songs: marching songs, drinking songs, campfire songs, feast songs, all of them different and none repeated from one book to another (as far as I can remember). And they're not simple or hastily written: they're good songs, always fitting to the occasion in the story, rhyme and rhythm that make sense, some of them long with many verses, pleasing to the ear, and memorable. It must be over a decade since I touched Marlfox, but I can still recite "Seven Seasons Gone" word for word.
Did the author have any kind of inspiration, basing his songs on specific real-world ones? Obviously they're not direct transcriptions of real songs, since many of them include references to things or people specific to the Redwall world, but they might still be strongly based on songs Jacques heard in real life, maybe during his time as a sailor. If they're truly original and penned from scratch by Jacques himself, then he deserves credit as a poet/lyricist as well as a novelist.