An electronic book system is configured to allow a user to read a primary portion of an electronic book, then switch to reading a secondary portion of the book, the secondary portion being presented based on a current reading position for the main portion. A reader can repeatedly switch back and forth between primary and secondary portions without losing track of the reader's progress through the book. As an example, a reader in a textbook can immediately switch to a problem set corresponding to the reader's current position in the textbook.