The Voltaire bio by Orieux - it's stylishly written but also very of its time (would probably come across as mannered if encountering this style at first), and above all, really long (over 1000 pages at least in the German translation), so I'm not sure I'd recommend to you for French practice. If you want to read something Voltairian themed in French, have you considered the Fritz correspondance in the original? This has the advantage that Mildred already did the English translation alogorithms for you to counterchek if you get stuck.