I almost had a baking theme going with A Birthday Cake is No Ordinary Cake but then got stuck. An art poem seemed likely with Lots of Spots (Lois Ehlert), Masterpiece (Elise Broach), and The Art Lesson (Tomie de Paola). They did not fit together. The book theme seemed most obvious (though definitely not creative). I also attempted titles by the same author, titles published in the same year, spines with similar colors, and haiku form/syllables. They are works in progress.