Complex sentence: The Morgan book of hours is the best preserved of the surviving black manuscripts, in part because of its unusually thick leaves, which protect the lettering from the abrasive effect of chemicals in black dye when reacting against those in the parchment.
Split into simpler sentences: The Morgan book of hours is the best preserved of the surviving black manuscripts. Its unusually thick leaves protect the lettering from the abrasive effect of chemicals in black dye when reacting against those in the parchment.