It was probably appropriate for elementary-school kids who are more advanced than picture books but not quite at the novel stage. It was a paperback with illustrations that ran to the edges of the pages. They were cartoon- or comic-like illustrations — line drawings, very brightly colored, like the illustrations in the Where’s Waldo? or Magic Schoolbus books.

The plot had something to do with a pair of children, boy and girl, going into outer space with various cartoon alien species. But I remember the plot less than the concept of the book, which was that every page or pair of open pages had a puzzle or a mystery you needed to solve, based on observation and deduction. You didn’t have to solve the puzzle in order to understand the rest of the book; it was more of a fun brain-teaser. The illustrations were very detailed, so there was a lot to look at–lots of clues, lots of red herrings.

The puzzle I remember the most clearly is that on one page, the children arrive at a space port with some alien friends. To pass through the port (to get to the rest of the alien world, or the rest of the spaceship, or whatever), the children and their friends have to enter a door or doors that will accommodate all of them. The illustration shows that the port has a whole series of doors in a row, and above each door is a schematic showing who may pass through it. One door, for example, may only allow a pairing of one round alien and one tall skinny alien. Another door may allow three boxy-shaped aliens and a human in a spacesuit to enter. There are lots of aliens of all shapes and sizes roaming around in the illustration, so it’s clear that specific combinations of shapes and numbers of aliens enter through specific doors. The key to the puzzle is to look at the drawings above the doors and figure out which door or combination of doors is permissive enough to allow both the children and their alien friends to pass through.

The rest of the puzzles are like this. Based on what the text says, and what’s in the illustration, can you find something the children need? Can you figure out where they need to go? That sort of thing. Some of the puzzles may have needed some basic math or science skills, but I think most were self-contained observation/deduction brain-teasers. If I recall correctly, the back of the book had answers for the puzzles, plus extra fun tidbits about various details in the illustrations.

I loved this book so much as a kid because it made you think and pay attention. I would love to give copies to all my friends and relatives who are now having kids, if I could only remember what it was called! Any help would be greatly appreciated.