More than a decade and a half ago, I have a recollection of a friend coming to school one day with a small book which was an early Magic the Gathering RPG. I recall that it was about being a planeswalker and some other minor details. I can't for the life of me remember anything more about it, or find it on ebay. Does anyone know anything about this game? My interest is simply for nostalgic reasons.
If your timing is right, and your terminology close, then what you might have seen is an Everway sourcebook. Everway was a card-based roleplaying game released by Wizards of the Coast concerning people known as spherewalkers who can journey from world to world. It could easily be mistaken for a Magic: the Gathering RPG.
To be clear, however: no official Magic RPG exists, or has existed. Wizards of the Coast's marketing department has enforced a strict silo between the TCG and RPG development divisions.
EDITED TO ADD: Ryan S. Dancey has, on Reddit, posted what can be seen as a comprehensive supplement to this answer.
Other Wizards of the Coast employees chime in on his Facebook page.
There was a WotC published rulebook for MtG, which was rather scarce. I think I still have one or two copies lying around. It could be mistaken for a roleplaying manual.
There also were some early books published (The Arena) which had a mail-off coupon for free card(s). Which were written as novel(s), but could be viewed as a roleplaying sourcebook.