What is the author's tone in The Rebel by Albert Camus?

It must first be noted that the book was originally written in French and translated, meaning that it may not be possible to get a full and accurate sense of the author's original tonal intent. That said, and as suggested above, the overall tone of the book is one of intellectual discussion, of a reasonably objective injection of new information and/or perspective into an ongoing debate. If the reader is a member of the intended audience described above, this tonal quality could very well be engaging, effective, and above all thought provoking. For readers who aren't well-educated fellow philosophers and/or intellectuals, the language and ideas, as well as the way they are presented, could very well be more than a little overwhelming.

In that context, it's interesting to note that in the book's final sections (4 and 5 - "Rebellion and Art" and "Thought at the Meridian"), the author (and presumably the translator) allow a slightly different tonal quality to enter the writing. Where the early sections have been, as suggested, quite objective and unemotional, an undercurrent of passion begins to make itself felt in these final sections. It never becomes overt or overwhelming, but there is a certain sense that here the author believes what he's saying and feels strongly about it, whereas before there is the sense that he has considered what he's saying and has thought deeply about it. There is a personal, spiritual, emotional connection to the ideas in these sections not present in the earlier chapters. While the earlier lack of evident feeling isn't automatically a bad thing, there is certainly the sense that those earlier chapters in some sense presented "the problem," while the passion in the final two sections carries with it the suggestion of the author's faith that the ideas there present "the solution." That solution might be summed up in one word - hope.