Elias started reading at midnight. Chapter 1: Sets. “A set is a collection of objects, but beware: not every collection is a set, lest we wander into the paradox of the barber who shaves all those who do not shave themselves.” A harmless footnote. He smiled, underlined it, and turned the page.

To give you a flavor of what awaits in the PDF, here is a typical problem from the "Series" chapter (paraphrased):