Susan at Chicken Spaghetti has a post* with links to Discover Magazine's 25 Greatest Science Books of All-Time, what the magazine calls "the essential reading list for anyone interested in science"; at the Discover page, you can also find a link to an essay by Nobel laureate Kary B. Mullison on the greatest science books.
Worth mentioning that the books on the list are all for adults. Which makes me wonder about which titles you would put on a "Greatest children's science books of all-time" list? I don't know that I'd limit it to 25, or even "great" -- I'd settle for very, very good and either "favorite" or "most useful", especially for home education. I'm pressed for time right now (trying to get to a hay ride at 5 pm), so I'll post my suggested titles later.
*In the same post, Susan also offers a wonderful roundup of all the recent lists of the best in children's literature for 2006. Thanks, Susan.
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I'll go first:
How to Think Like a Scientist: Answering Questions by the Scientific Method by Stephen P. Kramer with illustrations by Felicia ("If You Give a Mouse a Cookie") Bond