Sylvia Engdahl (
ISFDB) is offering 50% off two of her vintage YA SF novels published during the 1970s:
This Star Shall Abide (1st in a trilogy also in a
3-book omnibus edition, not on sale) and
Journey Between Worlds which is a standalone ($1.50 each after couponage)
ETA: She also offers for the same price a vintage YA historical biography written by her late mother about Katherine of Valois (wife of Henry V of England), which the blurb says was accepted for publication by Houghton Mifflin Harcourt in the 1970s and edited, but then the editor left before it was published and her replacement discontinued the series:
The Disobedient Queen: Katherine of Valois