I really do not understand why "MaddAddam" was written. This does not mean I did not like the book, but I have to wonder why this book was written. I can only conclude Ms Atwood likes the characters and needed some closure.
The events in "MaddAddam" take place after the end of "Oryx & Crate" and "The Year of the Flood". If you read the two above you will want to read "MaddAddam" too and while the back story is interesting it could have (should have?) been included in the first two books.