I've avoided this thread all week - just in case. Just had a chance to watch the final episode tonight. I thought it was so well done - can't wait for season 2 and I admit, part of me wondered if it would be a disappointing ending to set it up for the next one.
I haven't read the book - might do so now if this is us at the end of the story as in the first book?
* Warning - The following contains potential spoilers for both the final episode and the book. *
The very end of the TV series is essentially the same as the book, i.e. Offred being driven away by the "Secret Police". Offred's last speech as she steps into the van is lifted directly from the book. One slight difference in the book is that as well as telling Offred to trust him, Nick also tells her that the men are with "Mayday", i.e. the resistance. However Nick's intentions and affiliations remain uncertain, so Offred's future is too.
The book's ending is deliberately ambiguous, and no sequel was intended. The "epilogue" I referred to above is an academic discussion set further in the future at a "Symposium on Gileadean Studies" in 2195. Two of the academics have discovered Offred's story on cassette tapes and transcribed them as "The Handmaid's Tale".
The TV series has kept pretty faithful to the book. The main difference is that the book is written entirely from Offred's perspective, so the TV series has taken the opportunity to expand on things she knows nothing about, e.g. what happened to Luke. I think it's fair to assume that the TV writers will take full advantage of the book's ambiguity to go even further and create their own interpretation of what happens after Offred enters the van.