You're concerned that when the reader looks for the solution they will see other solutions.
The easy answer would be just put one answer per page, but that would make the book absurdly long. So instead, don't put the solutions in order, or with a key that they could recall back to the question.
In other words, Don't label the answer for question 50, with 50. Label it with "Apple", or "Purple" or "Rhino". That way, if/when they see other answers there is no way to say, "The answer to question 50 is 'A clock'." It's, "The answer to 'Beetle' is 'A clock'. But I have no idea when I'll see question 'Beetle'."
The code words for answers are in alphabetical order so they are easy to find, but the order in which they match questions is completely random: 1 is 'Dog', 2 is 'Orange', 3 is 'Banana', etc. You should never have the the answer to any of the previous 5 or next 5 answers on the same page.
Hide the clues in plain sight
Way back when, Infocom (a computer game company) released hint books called InvisiClues. They were meant to help the player solve puzzles by using a special pen to expose the invisible ink to give more and more detailed clues until finally revealing a complete solution. The riddle books could have something similar in that a single page would hold the riddle, then below it would be checks to see if they got the right answer; "It's a five letter word", "It starts with a C", "You probably have seen a few of them today", "The answer is, a Clock."
If you'd prefer a non-destructive method (using the pen, once revealed, you can't re-hide a clue), you can use cellophane filters. Also long ago, there would be contests where the "prize" was hidden in a mashup of multi-colored dots. Looking at it, you could not read what the prize was. You would need to go to a store's kiosk and they would have a filter setup so it would alter the color you perceived for the dots and could read what you won.
The affect is called anaglyph.
You could do something similar for the answers where the reader could expose either a clue, or part of an answer. I'm sure with technology you could even create answers that show different words with different color filters; a red filter shows a clue, a blue filter shows a different clue, but when you use a red and blue at the same time it shows the answer. Similar to National Treasure.