Online engineering/scientific books (like this one) need interactive tools/examples. I use JavaScript libraries such as JSXGraph, MathBox and plotly.js to create interactive examples. Some are as follows.

Math

• Eigenvectors and eigenvalues of 3-by-3 matrices
• Change of basis in 2D
• Change of basis in 3D
• Rotation in 2D
• Rotation in 3D
• Rotroinvestion
• Eigenvalues and transformation of a circle
• Eigenvalue and transformation of a sphere

Static and solid mechanics

• Simple truss analysis
• Simple beam analysis
• Strain measures
• Stress tensor
• Principal stresses in 3D
• Maximum shear stress in 3D