Learn Spanish (Latin America): La encuadernación protege el lomo del libro. - The binding protects the spine of the book.

Yes! It essentially means the process of turning pages into a cuaderno (notebook or bound book). The prefix en- (into) and the suffix -ción (action/effect) surround the root to mean 'the action of making into a book,' which gives us 'binding.'

I've seen lomo on restaurant menus. How does it mean 'spine' here?

Lomo generally refers to the back of an animal, which is why it translates to 'loin' on meat menus. By visual extension, it is also used for the 'back' of a book, which we call the 'spine' in English. Note that a human's anatomical spine is the columna vertebral, not the lomo.

Why does protege use a g instead of a j for that breathy 'H' sound?

The infinitive is proteger. In Spanish, the letter g naturally makes that breathy 'H' sound when it comes right before an e or an i. You only need to switch to a j to maintain that sound if the following vowel is a, o, or u (for example, in the 'yo' form: protejo).

Tying into this lesson, if the subject was 'La mayoría de las encuadernaciones' (The majority of the bindings), would the verb be singular or plural?