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

The 21st Century Skills Library: Animal Invaders series by Susan Heinrichs Gray is at a Grades 4.9–5.9 reading level (average Grade 5.4). Intended for Middle Grades (Ages 9–12). There are 7 books in the series.

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

The 21st Century Skills Library: Animal Invaders series has 7 books in order: 1. Bullfrog, 2. Emerald Ash Borer (Animal Invaders), 3. Giant African snail, 4. Starling (Animal Invaders), 5. Termite, and 2 more.

21st Century Skills Library: Animal Invaders Books in Order

7 books by Susan Heinrichs Gray. Reading level: Grades 4.9–5.9. Middle Grades (Ages 9–12).

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 reading level Grades 4.9–5.9, with average content intensity 1/5. Intensity stays consistent across the series — a kid who can handle the first book can generally handle the rest.

The reading-order table below lists every book with per-volume reading level and intensity badges 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.

Books

Grades 4.9–5.9

Reading Level

1/5

Avg. Intensity

Middle Grades

Content Intensity Across the Series

Consistent

Content stays at a steady intensity level throughout the series.

First book Latest book

21st Century Skills Library: Animal Invaders Reading Order

#	Title	Reading Level	Content
1	Bullfrog Susan Heinrichs Gray	Level 5-6	Gentle
2	Emerald Ash Borer (Animal Invaders) Susan Heinrichs Gray	Level 5-6	Gentle
3	Giant African snail Susan Heinrichs Gray	Level 5-6	Gentle
4	Starling (Animal Invaders) Susan Heinrichs Gray	Level 4-5	Gentle
5	Termite Susan Heinrichs Gray	Level 5-6	Gentle
6	Walking catfish Susan Heinrichs Gray	Level 5-6	Gentle
7	Wild boar Barbara A. Somervill	Level 5-6	Gentle

All 21st Century Skills Library: Animal Invaders Books

Bullfrog

Susan Heinrichs Gray

Bullfrog

Susan Heinrichs Gray

Level 5-6 Gentle

Emerald Ash Borer (Animal Invaders)

Susan Heinrichs Gray

Emerald Ash Borer (Animal Invaders)

Susan Heinrichs Gray

Level 5-6 Gentle

Giant African snail

Susan Heinrichs Gray

Giant African snail

Susan Heinrichs Gray

Level 5-6

Starling (Animal Invaders)

Susan Heinrichs Gray

Starling (Animal Invaders)

Susan Heinrichs Gray

Level 4-5 Gentle

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

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

What reading level is the 21st Century Skills Library: Animal Invaders series?: The 21st Century Skills Library: Animal Invaders series by Susan Heinrichs Gray is at a Grades 4.9–5.9 reading level (average Grade 5.4). Intended for Middle Grades (Ages 9–12). There are 7 books in the series.
What order should I read the 21st Century Skills Library: Animal Invaders books?: The reading-order table above lists all 7 books with per-volume reading level and intensity ratings. Start with book 1 and read in publication order unless the table indicates a different recommended order for newer readers.
What age is the 21st Century Skills Library: Animal Invaders series appropriate for?: The 21st Century Skills Library: Animal Invaders series is recommended for Middle Grades (Ages 9–12). The average content intensity is 1/5. Check the intensity trajectory above to see whether content gets heavier across the series — if it does, sensitive readers may want to stop earlier in the run.