You know the child is turning seven and you have about four minutes to decide. You are buying exactly one book here. Find the age, buy that book, wrap it, done. It runs youngest at the top to oldest at the bottom, so scroll to the birthday and stop there: there's a two-year-old's book and a twelve-year-old's book and six in between.
It's a shelf of eight birthday presents laid out youngest to oldest, and the only question you need to answer is how old they're turning. Two and three: Press Here, which is the one people give when they want a present that feels like magic. Four to seven: Dragons Love Tacos or The Day the Crayons Quit for laughing, Last Stop on Market Street for a family who likes a warmer, quieter book. Six to nine, especially a child who says they hate reading: Dog Man. Seven to ten: The Bad Guys. Eight and up, for a child who reads: The Wild Robot, then The Lightning Thief at nine and above. Buy one. Each is a whole gift by itself, and nobody will ever know there were seven others.