The "My Spheres by Sequence" view in the Spheres Tab has notes fields for Shadow, Gift, Siddhi, and Line. Enter notes while reading the books.
Spheres > Sphere Notes is a place to take notes about the Spheres themselves. There are a few other notes fields scattered about.
Several of the views contain Attachments fields where you can upload images, PDFs, videos, etc. Use these as you wish.