I checked this book out multiple times from our public library. Author’s name must
begin with Bu-Ea based on shelf location (preceded Eager; after Burnett), is not
Ruth Chew after reviewing her catalog. Previous helpful respondents from other attempted search sites have suggested A Walk in Wolf Wood
(Mary Stewart) or A Walk out of the World
(Ruth Nichols), which are lovely and similar, but incorrect (my lovely childhood public librarians were not illiterate enough to repeatedly mis-shelve this book, and my memory is solid about where it was located on the shelves).
This is a beautifully illustrated, middle-reader American (I think) fantasy book about a brother/sister or possibly male/female friends who somehow stumbled from their somewhat urban home/apartment building into a medieval setting when coming home from school one day in the woods near their home.
Sister ends up spending time with women from the group (I remember some looming?) and brother with the men (possibly becoming a page/squire?) There is maybe a joust, some magic or Merlin-y magician, the illustrations were very lovely
with lots of woods/trees. Cover art and some interior pen+ink drawings reminded me of the cover to the film adaptation of Camelot.
I was born in 1970 and was a very early and voracious reader, so this book cannot have been published much later than 1976; my guess (contextually) is that it’s most-likely a little earlier than that.