This book is meant to make fun of Leibniz. It's fairly funny and I enjoyed it as a teenager, before I knew anything about Voltaire or Leibniz or most of what this book talks about. Teenage me just enjoyed the raw sarcasm and smugness.
Leibniz has a spotty record as a philosopher, but his contributions to mathematics go far beyond the calculus you normally hear about. His fingerprints are all over pure math and formal logic and computer science and computer hardware and engineering in general. Next time you're in an airplane and it doesn't crash, thank Leibniz twenty times over.
Voltaire, on the other hand, was a rich, sarcastic dickhead whose rants against Jewish and Black people were extremely racist even by the standards of the 18th century. Screw this book.