Castle in the Attic

by Elizabeth Winthrop

It's been a lot of years since I read this one, so I'm copying and pasting the summary from wikipedia, but I loved this book as a kid.  As soon as the reserve comes in from the library I'm reading it again.

William is given a realistic model of a castle by the housekeeper, Mrs. Philips, who tells him that it has been in her family for many many years and that its silver knight is said to be under a spell. The silver knight, Sir Simon, comes to life and tells William stories about olden times and an evil wizard who is ruling his kingdom. Desperate to stop Mrs. Philips from going away, William has Sir Simon shrink her with a magic token he stole from the wizard, Alastor. However, William and Sir Simon lack the ability to return Mrs. Philips to her true size as the half of the token that can do so is with Alastor, and Mrs. Philips falls into a depression. Learning of a legend that states that when there is a lady, a knight, and a squire, a quest can be undertaken to stop Alastor, William decides to become a squire to undo his mistake. As he will be shrunk willingly, he will return to his world at the exact moment he left, but Mrs. Philips will lose all the time she spent in the castle until William enters. William has Sir Simon shrink him and he enters the castle to join his two friends.

Audience
I think once you are a fluent reader you can read this (3rd grade or so)

What I Love:
This all makes sense!  There's a reason William is so good at medieval skills, and the magic is even logical.  It's easy to believe and the characters are all endearingly flawed.  I can barely remember the story, but I remember well how much I love Sir Simon.  This is one of those books everyone tells me gets better when you read 

Caution:
This is heavy medieval fantasy.  If it's not your cup of tea move on