Bick, I would take it that The Outsider/The Stranger is a "primary" work. It's not a scholarly commentary on a topic with the setting-out of context, the interpretation of closely-read passages, and so on, but a work in its own right.
A problem with secondary writing (critical commentary, etc.) is that it's often concerned to answer questions you've never had, to participate in a scholarly conversation that's been going on while you were doing other things, and so on. Many primary works may need some brief editorial helps -- identifying persons and places named in the book and so on; but the work will often be intelligible enough as not to require readers to sit down with a pile of commentaries.
I don't mean to disparage the whole genre of scholarly commentary. To take an example that will be familiar to many Chrons people, Tom Shippey's Road to Middle-earth is a splendid work of secondary writing.
Again, I haven't read enough philosophy to know, but my hunch is that many pre-modern philosophical works do not require the reading first off of whole books of secondary commentary. A good translation with some brief editorial notes suffices (I have found) for Plato, Augustine, Boethius, the Tao of Lao Tse, Sir Thomas Browne's Religio Medici etc. -- "suffices" in the sense that one can have a very worthwhile reading experience. I imagine the same would be true of Aristotle, Confucius, and many others. But I wouldn't be surprised if I'd need to read something like one of those Very Brief Introductions from the OUP if I were going to try to read Kant or later philosophers.
Speaking for myself, I'd generally rather read another primary book than, having read a primary classic, set myself to read a great deal of secondary commentary. I have a few books on Plato, but the only one I've read in its entirety was short.
In short, I think the majority of the world's great philosophical works were not written "only" for grad students and academic philosophers to read. Philosophy is now often taken to be a matter for academic specialists. But, as one of the young C. S. Lewis's friends said to him, and he never forgot it, Philosophy wasn't a subject to Plato; it was a way.