Twiggle Books are divided into 5 levels based on a combination of factors including Flesh-Kincaid readability scores, predictable text, word repetition, word count, word structure, illustration, vocabulary, and sentence structure. While remaining appropriate for early readers, the texts grow more complex at each level to facilitate increased reading vocabularies, fluency, and comprehension skills.

Circle Level Books at the circle level build early literacy skills such as print awareness and print motivation. Children can practice letter sound and word recognition skills as they enjoy texts with easy-to-follow predictable text patterns, picture clues, rhyme, rhythm, repetition, and simple sentence structures. 
square level Books at the square level continue to engage the interest and enjoyment of early readers with increased word counts, more varied vocabulary, slight alterations in sentence structures, and patterned text that is broken up at the beginning and end of the stories.
triangle level Fluency becomes a focus at the triangle level as word counts continue to increase and sentence structures contain more variation. Early readers, who can easily read books at the circle and square levels, are exposed to an enriched reading vocabulary that is made accessible through the use of rhyme, repetition, and rhythm.
star level Books at the star level continue to support literacy skills from the previous levels while challenging early readers to acquire categories of new reading vocabulary and increase fluency. The focus switches from a reliance on predictable text patterns and picture clues as aids to comprehension to the use of context clues and decoding skills. Books at this level also provide opportunities for students to use comprehension strategies such as sequencing and prediction.
diamond level Increased fluency continues to be the focus for early readers at the diamond level as texts become more complex with distinct beginnings, middles, and ends. Children have opportunities to build more extensive reading vocabularies in addition to practicing skills such as using context clues to determine the meanings of unfamiliar words, decoding words with complex letter combinations, and making predictions.