Short answer: Map the syllabus to a small set of books and previous papers, and avoid reading list inflation.
Map the syllabus
Divide the syllabus into units and attach one primary source to each.
Use previous papers
Previous papers show how the optional is examined, so use them to prune the list.
Keep the list short
Fewer books read twice beat many books read once.
Frequently asked questions
How many books for an optional?
A small, focused set mapped to the syllabus works better than a long list.
Can I buy optional books second hand?
Yes, check the edition and condition.