In December 2017, for no special reason I started studying mathematics and writing solutions manuals for some textbooks.
A solutions manual for Set Theory by Thomas Jech
GitHub repository here, HTML versions here, and PDF version here.
A solutions manual for Algebra by Thomas W. Hungerford
GitHub repository here, HTML versions here, and PDF version here.
A solutions manual for Topology by James Munkres
GitHub repository here, HTML versions here, and PDF version here.