Copyright risk of an educational website relying repeatedly on the same textbooks

I am reviewing the copyright risk of an educational website that publishes independently written articles explaining technical concepts, equations, and calculation methods.

Several foundational textbooks are used repeatedly as references. A single textbook may be cited in 20–25 separate articles within the same subject category.

The website does not distribute the textbooks or reproduce their figures, tables, or solved examples. Standard equations and technical terminology are retained where necessary, while the explanations are independently written. Bibliographic citations are provided. Passages that appeared to follow a source’s wording or explanatory sequence too closely have also been rewritten.

Could the website as a whole nevertheless be considered an unauthorized adaptation or a substitute for the textbooks because the same sources are used repeatedly?

Which factors are most important when assessing the risk?

• Similarity of wording
• Selection and arrangement of topics
• Following the textbook’s chapter or teaching sequence
• Reuse of distinctive examples
• Number of articles citing the same book
• Cumulative amount derived from one book
• Whether the website is monetized

Is repeated citation of the same textbook itself relevant, or is the main issue whether the protected expression, examples, or original structure of the book have been reproduced?