The plot is a bit like Childe Rowland and/or ...

[Richard]

Childe Rowland is an English folk/fairy tale that goes way back. Shakespeare references it in his King Lear. The story is that a girl disappears (abducted by the fairies) and that her brothers attempt to rescue her. Childe Rowland (which means young knight Rowland) succeeds. This involves Merlin and some head-chopping-offs of innocent strangers and a door which must be opened.

Of course, I see traces of the story The Magicians of Caprona by Diana Wynne Jones in which two children are abducted by cursed books which they receive somewhat surreptitiously. Also, DWJ's story features some magically hanging banners warning the parents/families to desist from certain actions, e.g. "foot-high letters, which seemed to hang in the air in the middle of the yard. In the fading light, they were glowing an unpleasant, sick yellow. STOP ALL SPELLS OR YOUR CHILD SUFFERS. CASA PETROCCHI"
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So at last her eldest brother went to the Warlock Merlin and told him all the case, and asked him if he knew where Burd Ellen was. "The fair Burd Ellen," said the Warlock Merlin, "must have been carried off by the fairies, because she went round the church 'wider shins'--the opposite way to the sun. She is now in the Dark Tower of the King of Elfland; it would take the boldest knight in Christendom to bring her back."