The book is available from Audible (audio) and Amazon. The Maze of Bones is the first book in The 39 Clues series. It is written by Rick Riordan, and was published by Scholastic in 2008.
Amy and Dan Cahill, the protagonists of the novel and Grace's favorite grandchildren, are on their way to Grace's funeral after her death. Her will proposed a challenge: to find the 39 Clues, which will make the winner the "most powerful and influential person(s) on the planet", or to take a million dollars and walk away.
They accept the challenge and they received the first Clue. They find a lead in Philadelphia, prompting them to travel to Paris, France, where they learn more about Ben Franklin (who also supposedly a Cahill) and their family.
The almanac they found mentions a 'maze of bones'. They realize this means the Paris Catacombs, whose walls have been paved with bones. They find some bones which have numbers on them: a number game, planted there by Franklin to give the coordinates to the next clue.
Dan solved the first clue which means a type of metal: iron solute. The probable location of the second Clue is Vienna, Austria, the home of Wolfgang Amadeus Mozart.
Investigating the art of the audiobook - Audible founder reports 40% growth in unit sales every year http://goo.gl/q1Iqh