Here are some of our favorite science books. Though we are not yet middle or high school level at our house, I have compiled some I think you might like. I have also included some traditional texbooks from the 21st century. They are categorized by a broad range below. There are many more on Google Books. You can use some of the words in these titles as key words in your search. You can read more about searching for free ebooks on Finding Free Textbooks on Google Books or Searching the Internet Archive.
Primary
You can find more science for primary in Narrative Style Science Readers
Plant and Animal Children: How They Grow
Intermediate
Object Lessons in Elementary Science: Following the Scheme …
First Year Science-with experiments
Real Things in Nature: A Reading Book of Science for …
Science for Beginners: A First Book in General Science for … with exercises to illustrate principles
Science for Beginners with exercises to illustrate principles
Middle Grades
CK-12 Life Science for Middle School
CK-12 Earth Science for Middle School
From Vitamins to Baked Goods: Real Applications of Organic …
High School
Zoölogy for High Schools and Academies
CK-12 Basic Physics – Second Edition
CK-12 People’s Physics, Version 3
CK-12 Earth Science for High School
CK-12 Chemistry – Second Edition
General Science: First Course with questions and excercises
The Good Goat Momma 