Reddit mentions of Reality Is Not What It Seems: The Journey to Quantum Gravity

Some general searches through the Phil Sci archives might turn up papers related to your interests here. In particular, there might be something under the general topics of cosmology, quantum gravity, or relativity theory.

Within that archive, some that stand out to me in relation to your question are: this paper on anthropic reasoning about many worlds, this paper summarizing unification arguments for string theory, this paper on the ethical implications of many worlds, this paper on general trends in philosophy of physics (one of which is the cosmological many worlds), this paper on whether or not string theory posits mereological simples that are extended, and this paper on what is involved in deriving GR from some string theories.

Some theoretical physicists who work on string theory or quantum gravity in general and who come to my mind as conscious of history & philosophy of science as well as metaphysics are: Lee Smolin, especially his books The Trouble with Physics and Three Roads to Quantum Gravity; Sean Carroll, perhaps even his lectures on time and his book The Big Picture; and Carlo Rovelli, especially his books Reality is Not What it Seems and Quantum Gravity. Again, I don't mention them to point to philosophical work on those topics but only to mention some physicists who work on those topics and who have a more philosophical bent.

I know Alexander Blum has looked into the history of quantum gravity but I don't know what he's written on the philosophy of quantum gravity or specifically on string theory. Tim Maudlin and Craig Callender do quite a lot of work on philosophy of space-time but I don't know that they have specifically discussed string theory. In general, you might find some interesting papers in the philpapers browser for the philosophy of string theory or of cosmology.

Also, Jeffrey Barrett has done quite a lot of work on quantum interpretations and Everettian many-worlds, which is not to be confused with the string-theoretic landscape of many worlds but you might find some of his work interesting.

Hope that helps!