A Big Book of Science adds one point to your science skill, or two if you have comprehension perk. There are 25 in the game, but one respwans every 72 hours in game on a scientist at Shale Bridge. It weighs two pounds like all other books, but is much thicker then the rest, so in my opinion it should weigh four pounds.
Follow link for location of this book and others. http://fallout.wikia.com/wiki/Fallout_3_books