Eric Cressey has an excellent lengthy article on things you should be aware of when doing pull-ups/chins as it might cause you problems
Jen Sinkler has an Q & A with John Broz, who is a coach teaching the Bulgarian system “meaning his athletes lift super heavy, super often — as in twice a day six days a week, and once on Sunday”
Bret Contras has posted 10 rules about progressive overload. It is well worth a read as there is more, well more than one way to do progressive overload.
For the next link I was tempted not to write anything but just let it stand on its own. I reconsidered. The workout in question is from when he was at the top of his carreer and should obviously not be attempted by beginners.