Reading code just for fun was a thing in the early and mid years of UNIX (eg., 7th edition or System V). People eagerly passed around faint 10th generation photocopies of Lion's printout of and commentary[1] on the UNIX source code. The annual USENIX conference had a popular short course in which they went through the entire UNIX kernel line by line. Why was that a thing back then and not now? Certainly UNIX was an a…
I read through the Lions book in the late 1990s when the reprint came out, essentially "just for fun" -- it remains probably the only sizeable piece of code I've read for fun.