It's been 25 years since I read it, but upon reviewing the plot summary for The Handmaid's Tale in Wikipedia:
The Handmaid's Tale is structured into two parts, night and other various events. This novel can be interpreted as a double narrative, Offred's tale and the handmaids' tales. The night sections are solely about Offred, and the other sections (shopping, waiting room, household, etc.) are the stories that describe the possible life of every handmaid, though from the perspective of Offred. In many of these sections, Offred jumps between past and present as she retells the events leading up to the fall of women's rights and the current details of the life which she now lives.
It seems like all the source material may have been used in the first season of the 2017 television production.
Are there any remaining plot points in the novel that remain to be revealed in season 2 and beyond? Or must the continuing series rely on extrapolations from the worldbuilding?
I should add, the novel contains an epilogue from the far future that purports to look back on the historical context of Gilead and touches on what happened later sociopolitically. It's very light, but it does reason about how Offred's story was recorded and became available to the world of the future (i.e. on cassette). I can assume that this might eventually come to pass in the TV series, but I'm asking about the primary timeline and its flashbacks, not what is inferred in the epilogue.