These are drawings of the Horizontal Format Bookmount but the Upright Format has exactly the same three modes of adjustability.
1. Book support plates pivot, or 'butterfly', on hinges to hold books at any opening angle:
2. Left and right leg pairs can be raised and lowered, insuring that the plexiglas support plates are snug against the book covers:
3. Support plates also slide laterally to accommodate varying spine thicknesses: