Fantasy Book: Time in the Fantasy Realm Passes Different from the Real World

Question

I am looking to identify a series of fantasy books for young readers, which I was aware of in the mid 1990ies.

One event I remember was that the main character, a pre-teen boy, would travel from our world into this fantasy realm and then return. After some time in our world he would travel to the fantasy world again. When he returns, he finds out that time has passed much faster in the fantasy realm, and a girl he was friends with the first time is now at least a teenager, and no longer interested in him. I clearly remember his frustration.

Each of the boy’s visits may have been the content of an individual book. I do not know how many there would be in total. The main story of each book would be some adventure that the boy gets involved in (none of which I remember).

The access point / gate to this fantasy realm may be in the boy’s grandparent’s garden. And the time between visits may be motivated by him only being at his grandparent’s during the summer holidays.

Answer 1

There is some similarity here with the Archives or Anthropos (1978-2001) series by John White. For some reason ISFDb only lists books 1, 3 and 4; Goodreads lists all 6.

The hero of the first 2 books, The Sword Bearer and Gaal the Conqueror is John Wilson, who has just turned 13 at the start of the first book. On his first visit to Anthropos he does leave from his grandmother's house, but in the second book he travels from a frozen lake.

In the second book he journeys to Anthropos to rescue Eleanor, the daughter of a former friend of his father, who has fled her father's threat to kill her. She travels to Anthropos a few hours ahead of John, and by the time he follows her several years have passed for her. (Time in Anthropos is apparently passing several thousand times faster than the real world; John's previous visit, the year before, is thousands of years in the past, though still recalled in legend.)

John is not the hero of the subsequent books, though; that role passes to his nephews and nieces. (He does play a somewhat larger role in book 5.)

Answer 2

I believe the series is the Chrestomanci books by Diana Wynn Jones, and the book with the young boy is The Lives of Christopher Chant, published in 1988.

Quoting from the Wikipedia page, these are the aspects that make me think of your book:

[The books' setting is] a multiverse often called the Related Worlds, which young Christopher calls "Anywheres" in this volume. Worlds have branched from common ancestors at important events in history, such as English and French victories in the Battle of Agincourt. Some people can move between worlds and twelve series of similar worlds have been labelled by the English people of the stories.

The only escape that Christopher has is through his dreams, in which he is able to escape to other worlds. While he is not the only person with this ability, seemingly no one is able to do it so easily as he. The fact that he can bring things back from these "spirit trips" makes him immensely valuable to his Uncle Ralph, a scheming silver-tongued businessman. He is soon caught up in a series of "experiments," supposedly to test his talents. In reality, they are to fetch a series of highly illegal goods (from mermaid meat to dragon's blood), for sale at the highest prices on a magical black market. He is accompanied on these trips by Tacroy, a guide arranged by his uncle.

Christopher is sent to boarding school for a while, which means he can't see his uncle as often as before (I think the communication through dreams and via Tacroy still continues, though).

I think the girl who loses interest in Christopher is The Living Asheth, "the physical embodiment of a goddess worshipped in a world in Series 10."