"Which One's Stronger!?" Series This is a picture book series filled with a variety of characters and adventures. - Series Name: Which One's Stronger!? This is a set of 27 books! My child loved these and collected them, but their interests have changed, so I'm selling them. Since my child used them, some books have covers and some don't. I haven't checked every book for condition, so there may be individual differences in staining, etc. Please purchase only if you understand that these are used items. Thank you for looking.