The New Way Things Work by David Mcauley, DK's The Way Science Works, and The Number Devil for my eldest son.
Will, God's Mighty Warrior (and 2 other books from that series), 2 early reader books (one about sharks, one about armadillos), and Otis the Tractor for my youngest son.
Usborne's Starting Gardening for my niece.
I usually give books to everyone I buy for, but my other niece really needs clothes, and my nephew asked for WWII models to build, so I did those this year. If my 2 other nephews read, then I would buy them books, but they just want gift cards, so I give them something I know they will use. I've always given books in the past (aside from last year) and they never read them. I can't afford that - thankfully, they are beginning to pass them back to us for my eldest!