Complex sentence: Bastian asks about one of the books he sees, but Mr. Coreander advises against it; despite which, Bastian seizes the book, leaving a note promising to return it, and hides in the school's attic to read.
Split into simpler sentences: Bastian asks about one of the books he sees, but Mr. Coreander advises against it. His curiosity piqued, Bastian seizes the book, leaving a note promising to return it, and hides in the school's attic to read.