Hilchot Shabbat 5:13-14 (Reading intensely by a candle… when the candle is not in reach)

Why is it forbidden to place a vessel underneath a candle on Shabbat if you’re trying to catch excess oil, but the same is permitted if you want to catch the sparks? What sort of reading is allowed by the light of the candle, and does it matter if the candle is not within reach?