What reading level is the 21st Century Skills Library: Animal Invaders series?

21st Century Skills Library: Animal Invaders books are at a grades 4–6 reading level (average grade level 5.4), and HootRated recommends the series for ages 9–12. There are 7 books in the series.

What order should I read the 21st Century Skills Library: Animal Invaders books?

Series numbers aren't available for most 21st Century Skills Library: Animal Invaders books, so HootRated lists them by publication date: Emerald Ash Borer (Animal Invaders) (2008); Starling (Animal Invaders) (2008); Bullfrog (2009); Giant African snail (2009); Termite (2009); and 2 more.

21st Century Skills Library: Animal Invaders Books in Order

21st Century Skills Library: Animal Invaders is a 7-book series by Susan Heinrichs Gray for ages 9–12, at a grades 4–6 reading level, with gentle content (HootRated intensity 1/5).

Books: 7; By publication date
Reading level: Grades 4–6; Average grade level 5.4
Recommended ages: Ages 9–12; From HootRated age ratings
Content intensity: Gentle; HootRated 1/5

21st Century Skills Library: Animal Invaders Reading Order

Series numbers aren't available for most 21st Century Skills Library: Animal Invaders books, so they're listed by publication date (oldest first).

#	Title	Reading Level	Ages	Content
1	Emerald Ash Borer (Animal Invaders) 2008	Level 5-6	9-12	Gentle
2	Starling (Animal Invaders) 2008	Level 4-5	9-12	Gentle
3	Bullfrog 2009	Level 5-6	9-12	Gentle
4	Giant African snail 2009	Level 5-6	9-12	Gentle
5	Termite 2009	Level 5-6	9-12	Gentle
6	Walking catfish 2009	Level 5-6	9-12	Gentle
7	Wild boar 2009 · Barbara A. Somervill	Level 5-6	9-12	Gentle

Content Intensity Across the Series

Consistent

Content stays at a steady intensity level throughout the series.

First book Latest book

Series books grow with readers — but that's also the trap. Most series start at one reading level and one content intensity and drift upward over the course of the run. 21st Century Skills Library: Animal Invaders (7 books by Susan Heinrichs Gray) lands at a grades 4–6 reading level, with average content intensity 1/5. Intensity stays consistent across the series, so a reader who can handle the first book can generally handle the rest.

The reading-order table above lists every book with per-volume reading level, age and intensity ratings so you can spot any escalation before it catches your reader off guard. For a deeper dive into how we score text difficulty vs. emotional weight separately — and why series readers especially benefit from that split — see our methodology page.

All 21st Century Skills Library: Animal Invaders Books

Emerald Ash Borer (Animal Invaders)

Susan Heinrichs Gray

Emerald Ash Borer (Animal Invaders)

Susan Heinrichs Gray

Level 5-6 Gentle

Starling (Animal Invaders)

Susan Heinrichs Gray

Starling (Animal Invaders)

Susan Heinrichs Gray

Level 4-5 Gentle

Bullfrog

Susan Heinrichs Gray

Bullfrog

Susan Heinrichs Gray

Level 5-6 Gentle

Giant African snail

Susan Heinrichs Gray

Giant African snail

Susan Heinrichs Gray

Level 5-6

Termite

Susan Heinrichs Gray

Termite

Susan Heinrichs Gray

Level 5-6

Walking catfish

Susan Heinrichs Gray

Walking catfish

Susan Heinrichs Gray

Level 5-6

Wild boar

Barbara A. Somervill

Wild boar

Barbara A. Somervill

Level 5-6

Series Like 21st Century Skills Library: Animal Invaders: What to Read Next

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Questions about the 21st Century Skills Library: Animal Invaders series

What reading level is the 21st Century Skills Library: Animal Invaders series?: 21st Century Skills Library: Animal Invaders books are at a grades 4–6 reading level (average grade level 5.4), and HootRated recommends the series for ages 9–12. There are 7 books in the series.
What order should I read the 21st Century Skills Library: Animal Invaders books?: Series numbers aren't available for most 21st Century Skills Library: Animal Invaders books, so HootRated lists them by publication date: Emerald Ash Borer (Animal Invaders) (2008); Starling (Animal Invaders) (2008); Bullfrog (2009); Giant African snail (2009); Termite (2009); and 2 more.
What age is the 21st Century Skills Library: Animal Invaders series appropriate for?: HootRated recommends 21st Century Skills Library: Animal Invaders for ages 9–12. Content is gentle (HootRated intensity 1/5). Intensity stays consistent across the series, so a reader who can handle the first book can generally handle the rest.