In one of those weird coincidences, today at work I placed copies of both of these books on the 'recently returned & popular reads' stand. The cover for the more recent Season Light is very striking and then I noticed they both had the same lady in them, albeit with a modified bodice.
NB. The actual cover of Season of Light comes with a sticker (of course) saying:
"We think if you like PHILIPPA GREGORY you'll love KATHARINE McMAHON"