There's a book that make the claim that the Rambam did not believe in the 13 principles of faith. I believe the claim was made on the basis of what the rambam wrote in the moreh nevuchim and/or in other of his writings. Do you know anything about this? Thanks

Shalom!

Maimonides compiled the "13 principles of faith." At the end of the thirteenth principle, he writes that whoever does not believe in any of these 13 principles, is not considered part of the Jewish people, and will not have a share in the world to come, et al - ("Maimonides' Commentary to Mishna - Sanhedrin X). Thus, it is not probable at all that Maimonides would be so hypocritical as to write one thing and practice another.

You should know also that in Moreh Nevuchim, Maimonides clearly wrote things that contradict his other writings, but this was done because Moreh Nevuchim was written with a specific audience in mind, those Jews who were being led away from Judaism by the contemporary philosophers of the time.(For example, see what Maimonides writes in Moreh Nevuchim about the reason why the Torah prescribes animal offerings.)