I grew up on Agatha Christie and Mary Stewart books. I particularly love The Ivy Tree by Mary Stewart.
Description on Amazon (doesn't do it justice:
An English June in the Roman Wall countryside; the ruin of a beautiful old house standing cheek-by-jowl with the solid, sunlit prosperity of the manor farm - a lovely place, and a rich inheritance for one of the two remaining Winslow heirs. There had been a third, but Annabel Winslow had died four years ago - so when a young woman calling herself Annabel Winslow comes 'home' to Whitescar, Con Winslow and his half-sister Lisa must find out whether she really is who she says she is.
If she likes The Ivy Tree I can guarantee she'll want to read all the others.
There are a couple of MNetters I know who've written YA adult - one set in the future, another in the past. Each of them is really well written and tell a great story.
www.amazon.co.uk/Five-Wounds-Katharine-Edgar/dp/0993159214/ref=sr_1_1?ie=UTF8&qid=1446140717&sr=8-1&keywords=katharine+edgar Five Wounds by Katharine Edgar]]
Description on Amazon:
It is 1536. The north of England has boiled over into rebellion against Henry VIII and the rebels march south towards London, growing stronger by the day. Sixteen-year-old Nan Ellerton, sent home from her convent when the King's commissioners arrived, has been promised in marriage to a powerful lord. When both he and his son Francis become embroiled in the revolt, Nan must choose - help the rebels, even though it could mean paying the brutal penalty for treason, or betray her beliefs and risk eternal damnation. A stunning historical novel for teenagers from debut author Katharine Edgar, Five Wounds tells a story of adventure, passion and courage, set against the backdrop of the Pilgrimage of Grace.
Changed Souls by Harriet Truscott
Description on Amazon:
After the virus came, everything changed. Seventeen Âyear-old Lucille is left with just a pile of debt to remind her of her dead family and their failed farm. Working for the Mayor to control the spread of the disease seems like a chance to repay her father’s betrayal. Now, the city is under an authoritarian regime, incinerating bodies, rationing food and quarantining families. When Lucille is called to the Mayor’s own apartment to take away a murder victim, everyone but her seems intent on a cover-up. Then the only person she trusts disappears… Little by little, Lucille realises that the service she put her faith in is corrupt to the core. And now she must learn to make her own choices about what loyalty and betrayal mean.