Um... I don't see $/tab specified anywhere. Perhaps the distribution of "tabs" will just decide how they allocate their $1,000,000?
From my unscientific evidence, it seems like opening a tab is .1 trees and .1 books, and .05 vaccinations. Still at 0.0 people with clean water and 0.0 shelters.
It would seem like you can accrue up to your 250 a day. I don't know what happens when it pushes up against the million global limit.
So go ahead -- contribute to books, knowing that really you're just taking away from clean water and shelter. Merry Christmas!