Cornelia Funke's delightful novel The Thief Lord is definitely worth reading with attention. Some readers are drawn to the book's setting, the city of Venice with its mysterious locations and abandoned movie theaters in which the children make their home. Others enjoy the characters and the interactions among them. It can be a lot of fun to figure out the motives of various characters (like the Conte or Ida or Scipio, for instance) and to see how they develop (like Prosper).
Still other readers appreciate the novel's adventure-filled plot line that is interwoven with fantastic elements like the lion's wing and the magical merry-go-round that changes a person's age. Many readers enjoy following the twists and turns of the story and trying to figure out its mysteries as they read.
Finally, many readers learn lessons in loyalty, love, compassion, and honesty as they read The Thief Lord, and they are invited to reflect on important elements like morality, mercy, selfishness, conscience, and childhood/adulthood.
As for an attention-getter for an essay or other writing assignment based on this book, you might start with an especially striking quotation from the novel. Depending on what you are writing, you might use the following: "Children are caterpillars and adults are butterflies. No butterfly ever remembers what it felt like being a caterpillar."
Alternately, if you are trying to convince other people to read this book, you might begin with an appealing question, something like, "Do you like stories that catch your attention and hold it until the very last page?" Then explain why The Thief Lord is a book that does exactly that.