A research question

Roaming through the cluttered and dusty Rare Book stacks at your local university library, you come across a large folio volume bound in sheepskin. A gilt central ornament on the front cover catches your eye. On closer inspection you find a name “Gilbert Erle of Cassillis” embossed in a circle around the ornament. The volume was printed in Paris, 1557. On the title page the name Cassillis has been written; this is evidently provenance, and the hand looks contemporary with the imprint. Your local binding expert suggests that the binding is about the same age as the volume (certainly no earlier, and no more than 20 years older); it is typical of sixteenth-century French binding styles. For whom was this book bound? Why do you suggest this ownership?