The Design of Approximation Algorithms by David Williamson and David Shmoys, Cambridge University Press, 2011. http://www.designofapproxalgs.com/
Approximation Algorithms by Vijay Vazirani, Springer-Verlag, 2004. https://www.ics.uci.edu/~vazirani/book.pdf
Chandra Chekuri's Lectures. https://courses.engr.illinois.edu/cs583/fa2021/