Tim Button wrote up his Cambridge lecture notes for a course on set theory for philosophers, previously taught by Michael Potter and then Luca Incurvati, as Open Set Theory which he then contributed to the Open Logic Project, and the resulting Set Theory: An Open Introduction can now be downloaded from this page on the Project’s site. The result, as you would expect from this author, is very good. It makes an excellent, and free, alternative to e.g. Enderton’s famous introductory book. It is particularly clear in marking off the initial informal (naive) development of the theory of sets, cardinals, ordinals etc. from the ensuing elaboration of an official iterative conception of the hierarchy and the formal development of an axiomatization of ZFC. It is perhaps just occasionally uneven in level; but when I do a half-yearly update of the Teach Yourself Logic Study Guide in a few weeks time, this will get promoted to a top recommendation.
I’ll be checking out again more of Open Logic’s offerings, for this project seems to have developed very well. In particular, I’ll also be recommending Richard Zach’s Incompleteness and Computability, which you can download here. More in TYL 2020.5.