I'm having trouble understanding chapters 14 to 23, everything is moving so fast. Can you summarize them?
You are right. It DOES go quite fast. What you are asking for here is the summary for the second half of the book. Although there is not enough room here for an extended summary, you can find it (chapter by chapter) through the eNotes link below. I am happy to explore the chapters you mention in the meantime.
The death of Elizabeth's pet bird puts a new adventure into the Egypt Game. There is a procession complete with wailing! The little bird "prince" is eventually mummified and placed in his own tomb: a pyramid! Through all of this, the kids forget to hide the grandiosity of their game (all except for Marshall). They learn more about Egypt in school and create a stuffed-owl "oracle," hoping it will answer Ken's question about being a star. The oracle replies through paper messages (of an unknown author). Meanwhile, Marshall loses his stuffed animal octopus named Security. The kids ask the stuffed-owl "oracle" and then consider stopping the game, which is becoming too real. It turns out that Toby is pretending to be the oracle and answering the questions. Marshall is still hoping the oracle can help him find Security. The oracle does not disappoint, and this time it's not Toby! After an unexplained altercation with a mysterious being in an alley after a concert, April and Marshall are left injured. An employee at the store is named the culprit & confesses to the police. Marshall's bravery increases his confidence and "the professor" is revealed as the one who always watches the kids from his window. The professor tells the kids a story (HIS story) on Christmas Eve and gives them literal keys to the storage yard and their "magic land" so they can continue their adventures. Meanwhile, he is thankful that he has had an adventure of his own.
In conclusion, there is an air of adventure about the end of the novel when Melanie asks about the possibilities of exploring gypsies. It is a fun ending to a wonderful novel full of imagination.