Also see https://eileenmoynihan.wordpress.com/kids-and-teens-fan-club/

Books, in and of themselves, are magical doorways. Opening a book is as simple as opening a door in the real world, but often much more rewarding! Doorways within books are something special. A character who passes through a magical doorway into a world of adventure mirrors the adventure of the reader themselves. https://www.quirkbooks.com/post/most-magical-doorways-literature
Open Sesame! Ali Baba And The Forty Thieves – Arabian Nights
In the story, ‘Ali Baba And The Forty Thieves’ from Arabian Nights, Ali Baba discovers that he can open a door into a cave full of treasure by saying, “Open Sesame!”
Down The Rabbit Hole…Alice in Wonderland By Lewis Carroll
‘Alice was beginning to get very tired of sitting by her sister on the bank, and of having nothing to do: once or twice she had peeped into the book her sister was reading, but it had no pictures or conversations in it, `and what is the use of a book,’ thought Alice `without pictures or conversation?’
So she was considering in her own mind (as well as she could, for the hot day made her feel very sleepy and stupid), whether the pleasure of making a daisy-chain would be worth the trouble of getting up and picking the daisies, when suddenly a White Rabbit with pink eyes ran close by her.
There was nothing so very remarkable in that; nor did Alice think it so very much out of the way to hear the Rabbit say to itself, `Oh dear! Oh dear! I shall be late!’ (when she thought it over afterwards, it occurred to her that she ought to have wondered at this, but at the time it all seemed quite natural); but when the Rabbit actually took a watch out of its waistcoat–pocket, and looked at it, and then hurried on, Alice started to her feet, for it flashed across her mind that she had never before seen a rabbit with either a waistcoat-pocket, or a watch to take out of it, and burning with curiosity, she ran across the field after it, and fortunately was just in time to see it pop down a large rabbit-hole under the hedge.
In another moment down went Alice after it, never once considering how in the world she was to get out again.
The rabbit-hole went straight on like a tunnel for some way, and then dipped suddenly down, so suddenly that Alice had not a moment to think about stopping herself before she found herself falling down a very deep well.‘ https://www.cs.cmu.edu/~rgs/alice-I.html

Through The Back of The Wardrobe To Narnia – The Lion, The Witch And the Wardrobe By C.S.Lewis
I will always remember opening this book and going through the wardrobe with Lucy.
Other Magical Doorways Or Portals In Books – What Is On Your List?
- The Secret Garden by Frances Hodgson Burnett
- Through The Looking-Glass by Lewis Carroll
- Peter Pan by J.M. Barrie
- Mary Poppins by P.L.Travers
- Tom’s Midnight Garden by Phillipa Pearce
- Harry Potter Books by J.K.Rowling
- The Borrowers by Mary Norton
- Jumanji by Chris Van Allsburg
- The Hobbit and The Doors of Durin by J.R.R.Tolkien
- His Dark Materials by Phillip Pullman