We all really enjoyed this book!
First published in 1955, Sun Slower, Sun Faster is a fun book about time travel, but it's also packed with the living history that we love.
Alicia, at Love2Learn, wrote a great, comprehensive review of it a few years ago, so I'll just send you there for further details on plot and characters. As Alicia noted, details of the Catholic faith and even of the Mass are beautifully explained and interwoven with the suspense and history, as the young protagonists learn what the Catholic Church is all about.
And so, Sun Slower, Sun Faster is not just an adventure of time travel and fascinating events of the past -- it's the adventure of conversion.