It's fast-going. You may want to start with something more introductory, like: Levin http://discrete.openmathbooks.org/dmoi3.html http://discrete.openmathbooks

It's fast-going. You may want to start with something more introductory, like: Levin http://discrete.openmathbooks.org/dmoi3.html http://discrete.openmathbooks.org/dmoi3.html Hammack https://www.people.vcu.edu/~rhammack/BookOfProof/ https://www.people.vcu.edu/~rhammack/BookOfProof/ the early parts of Newstead https://infinitedescent.xyz/ https://infinitedescent.xyz/