Of course, there are many recent books that are sensational (by Jackie French, for example, or Catherine Jinks or Linda Newbery - or many others).
But my mind goes back to the books I adored when I was nine or ten, many of which hold up rather well even all these MANY decades later - and in which the historical research and detail are impeccable:
- Smith and Devil-in-the-Fog by Leon Garfield
- Carrie's War by Nina Bawden
- Rosemary Sutcliff's Romans in Britain books (or anything by her, really)
- Most of Geoffrey Trease.
(Yes, I may have mentioned these before, but that's because they're good.)
And as for the paranormal/time travel/weird stuff, it seems to me that it's still pretty hard to go past Penelope Lively, Ruth Park, L.M. Boston or Diana Wynne Jones. Or, for that matter, Beowulf.
I'm not suggesting the books of yesteryear will never be bettered, but if you write for younger readers, these are essential reading.
If you read books, or buy books for kids to read, don't forget the gems lurking in the backlists.
Even if they are ancient history. And only 150 pages long.