The link to CK-12 provides access to the master list of online resources in each subject. Flexbooks and downloads are available for customised resources extracted from these master lists for a wide range of variations on each subject.
The textbooks are in a format for reading online with each section hyperlinked and includes interactive features such as quizzes that reveal the answer and solution ones selected and practice quizzes that adjust the level of questions based on performance. The flexbooks can be downloaded for use offline in the offline reader, and can be customized. Many textbook compliations by various authors are also available in pdf versions.
While CK-12 is an online resource, they also have an offline library which all the material can be saved to for reading without internet too.
CK-12 Science Textbooks – OER Flexbooks
CK-12 has an amazing online resource of science texts, grade level from K-5 and by subject for middle-grade and high school.
CK-12 Online OER Science Resources K to Grades 5
CK-12 Online OER Science Resources Middle School
CK-12 Online OER Science Resources High School
CK-12 Online OER Maths Resources Grades 1 to Grades 5
CK-12 Online OER Maths Resources Middle Grades 6 to Grades 8
CK-12 Online OER Maths Resources High School Grades 9 and above
CK-12 also has some wonderful tertiary level (college/university) online textbooks too, and foreign language and ELA subjects, see more at their site, at https://www.ck12.org
A great number of the flexbooks available by CK-12 are also available as downloads in many formats, one place to find them in multiple formats is on the Internet Archive, – https://archive.org/search.php?query=CK-12