The Slave Dancer, by Paula Fox, won the Newbery in 1974. When I looked at some of its reviews on Amazon (fairly evenly divided between those that thought it was important and evocative, and those that thought it was overly depressing or boring), I noticed that they recommended it for ages 9-12.
I would not recommend this book for a child under 12 (although that may be The Slave Dancer's "reading level"), because the story is really quite brutal. It is not overly graphic, but it is emotionally very difficult - Fox does all too good of a job portraying casual cruelty amongst the sailors before the slaves are even purchased and loaded into the ship. After that, it's horror piled upon horror.
The Slave Dancer is basically a story about one boy's loss of innocence - the year is 1840, and 13 year old Jessie Bollier of New Orleans is kidnapped to serve as a fife player on a slave ship called The Moonlight, which sails to West Africa to pick up slaves, then to Cuba to trade the slaves for molasses, and then back to the United States. The black and white illustrations by Eros Keith suit the story very well (part of one is shown on the cover here, which I think is much more appropriate than some of the newer covers, which portray a totally misleading happy nautical scene).
From the first page, with its list of The Moonlight's officers, crew, and cargo ("98 slaves whose true names were remembered only by their families, except for the young boy, Ras"), and the note "Shipwrecked in the Gulf of Mexico, June 3, 1840: Survivors 2", I think it is obvious that this isn't going to be a happy adventure. Strangely enough, the story isn't overly intense, either (thus the many critics who said it was boring) - but I think her detached style actually adds a bit to the horror described. I think Fox's ending is pretty realistic, and it packs quite the emotional punch, but I don't want to give any more of it away than that - except to say that while it is not exactly a happy ending, it certainly isn't as depressing as what preceded it. It's a good ending, worth finishing the book to reach.
I can't say that I will ever number The Slave Dancer among my favorite Newbery winners, but I do think it is an important book. It was interesting reading this right after The Matchlock Gun, with its casual mention of slaves and man's inhumanity to man. A lot certainly changed in the years from 1942 to 1974.