I read several of his books earlier this years, including his "Dictionnaire Philosophique", and many of his letters.
Voltaire - Lettres Philosophiques (Flammarion, 1964)
The first one were his Lettres Philosophiques, a new approach of commenting on life and society without actually writing a philosophical essay, but rather easy to read comments on what was happening in reality. His exile in England allowed him to comment on the benefits of many aspects of the way the English state was organised, and implicitly comparing this to what was happening in France. He published his Lettres Philosophiques when returning from his exile, but the book was immediately forbidden and even burned on the stairs of the French parliament. In his Lettres, he praises the tolerance in England, the acceptance of different ethnicities and especially religions next to each other without repression. He also hailed the formal accounting the government had to show for the parliament.
Voltaire - Micromégas/Zadig/Candide (Flammarion, 2006)
Voltaire - Traité Sur La Tolérance (Gallimard, 2016)
In his "Treatise on Tolerance" he attacks the church and the justice system after the execution of Jean Calas, who allegedly had killed his son who wanted to convert to catholicism. Voltaire attacks the total inequality in the justice system, the torture, the manipulation of evidence, the influence of the church in judicial verdicts.
It is hard for us to understand the efforts that Voltaire made, including the writing of hundreds of letters to restore the honour of people he never even met, with the sole purpose of changing the justice system
In today's world, these books are still relevant. Whether in Iran, the United States, Russia, Israel, Syria and many other countries, Voltaire and his thoughts and actions are still needed. Stephen Pinker is advocating for "Enlightenment Now", and it's more than high time that the voices of obscurantism are overpowered again by the voice of reason.
What Voltaire means in today's world, cannot be overstated.