Very good idea! If you ask me it's totally worth to buy the light novel. Not only because to expand your personal collection, but it shows the story in other aspect.

To be honest I don't like the question generally "where to read the manga" if they finished the anime. From the very beginning! There might be differences in the manga, so you might know the story completely, not to mention if the story is good enough why not live it once more?

So, definitely worth it.