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As a child (late nineties, maybe early 00s) I read a novel about a slightly older girl protagonist (YA? there may have been a crush described but not central) who has a summer vacation adventure through a library which acted as a portal to other realms. I remember being pretty weirded out by the physics* of it and it not being quite clear what is and isn't real. The library had a courtyard with a juniper tree that appeared to act as a signifier for 'crossing over' but you could only see it through windows and not access it directly (and the juniper tree was personally symbolic to the protagonist as well - maybe she was called Juniper?). There was a new librarian, possibly called Miss Quiver or Quin or Quaver - something along those lines, probably, who acted as an ambiguous, prickly figure rather than a straightforward mentor/guide (and I vaguely remember a male/non-human semi-antagonist the the librarian is somehow allied with but also argues with). The protagonist eavesdrops and works things out and gets involved without direct guidance and with much sneaking around. At the start the weirdness is only at the library, and then it appears to bleed over, but that may just be in the protagonist's head. (view spoiler)[At the end of the summer the librarian leaves and school starts and she integrates socially and the (now unneeded) magic ends. (hide spoiler)]
There was a reason that home wasn't comfortable and the character didn't have a great social life; I think that they'd just moved because her parents were separating or something. I think no siblings, but am not sure of that. I guess set in 70s, 80s or early 90s - there aren't any computers or videogames, but it didn't feel dated otherwise. Probably USA/Canada - I think not British and not otherwise marked enough to have impacted). Hardback with the tree on the cover, I think. Taken out from Fish Hoek Public Library older children/YA section. (Cape Town)
*which is saying something, considering that I found the physics of e.g. The Amber Spyglass and Mostly Harmless quite logical and straightforward.
AR
Yes!!! That's it!! I have bought myself a second hand copy and knew it was the right one as soon as I read the first page :-) Thank you so, so, so much!!!!!!! I think the reason it was so important to me is that this is the first time I identified so completely with a character - a young girl who prefers to collect words than friends, loves winter and the solemness of shelves of books - and aspired to the other - all unknowingly, I recently went back to uni for a MSc in Info & Library Science!
(printed 89 in UK by Canadian author Melody Collins Thomason. Protagonist is called Junie, but then renamed Juniper Tree by the librarian/witch, (view spoiler)[ who wants her to help her get back in the garden that isn't real by serving as a substitute for the juniper tree that anchored the magic garden so that she can reconcile with her male lover who has been trapped in the garden and terrifies Junie. (hide spoiler)] Junie is lonely this summer because her best friend is away, (view spoiler)[ but she makes a new friend - Marcus and his dog Victoria. (hide spoiler)] (She is indeed living only with her mother, but her father left long before rather than recently). Junie first sees the garden in a painting in a new corridor of the library, and then again, without the tree, from 2 different windows (facing different directions) in Miss R. Quarterberry's house (bigger on the inside) (view spoiler)[ when she is invited for tea (hide spoiler)]. Miss Quarterberry cannot help her, because (view spoiler)[ that wouldn't work - to anchor the garden, Junie must find the way herself. (hide spoiler)]