filling some loopholes i also find this book handy for instance, you can start with the free KG scalar to know how things can lead to field decomposition, pp. 171, other thorough discussions on basic mathematical aspects of a quantum field like Feynman propagator
https://books.google.com.ph/books?id=7AX38Cqeg3kC&lpg=PR9&pg=PR9#v=onepage&q&f=false