Haha Ga Hakui O Nugu Toki (“When Mom Takes Her White Robe Off”) [English, 186 pictures], by Yokoyama Michiru

The other volumes basically follow the first book format where you have the main story for around 75% of the book and then 1-2 chapters of random MILF NTR. According to my little understanding of Japanese and the summaries it look like in the main story the NTR is straight and the mother will leave the son at the end for the doctor after tons of rape, gang rape, and mind break etc. some other characters show up like the son's girlfriend and the short hair female doctor in later volumes but they basically just add distraction to the main conflict between the son, nurse mother and that gigolo doctor

It's actually not to bad, as far as NTR goes. Mother is seduce by doctor. Dr. tells mother to have a fling with the broken leg patient(this goes on and off for awhile) meanwhile d. takes mother at any time for sex. Another nurse is introduce, is on to something from dr. and is later seen having sex with the dr. Dr right now screwing the mother and nurse. Son at this time goes into a relationship with younger nurse from hospital. During this time, mother is seen gang rape or its an illusion, not sure. Son and young nurse having and enjoying their good times. Son sees or notices mother;s relation or blackmail(w/e) with doctor. This is when son gets a little chaotic, attack mother(she refuses but gives in) Son goes a little wild and screws mother. They have sex for days non stop. Young nurse visits sons and catches them, she is appalled. Mother catchers her before she leaves and they have a threesome. This goes on for a little while. Mother still seeing dr, not much though.