J. Shewchuk: An Introduction to the Conjugate Gradient Method Without the Agonizing Pain
M. Gutknecht A Brief Introduction to Krylov Space Methods for Solving Linear Systems
Pluto notebook: pluto/julia, html, pdf (updated 2021-01-07 after introduction of abstol/reltol; Please ensure to have IterativerSolvers.jl v9.x installed.)
IterativeSolvers.jl (link updated 2021-01-07)