I would read Marcuwitz (https://www.jlab.org/ir/MITSeries/V10.PDF) for the impedance of posts and irises in waveguides and Mattaei Young and Jones (https://www.microwaves101.com/downloads/MYJ%20part%201.pdf https://www.microwaves101.com/downloads/MYJ%20part%202.pdf) for the design of post filters in waveguides.