Both Uses of

principal
in
The Grapes of Wrath

These things were lost, and crops were reckoned in dollars, and land was valued by principal plus interest, and crops were bought and sold before they were planted.†
Chpt 19

principal = most important; or person that is most important

And always they were in fear of a principal—three hundred thousand—if they ever move under a leader—the end.†
Chpt 19 *

Definitions:

(1) (principal)
most important; or person who is most important
The exact meaning of principal can depend upon its context. For example:
- "the principal idea" or "the principal performer" -- the most important
- "the principal of the school" -- the person in charge
- "a principal of a company" -- an owner or person with controlling authority
- "the principal in a transaction" (as contrasted to the agent) -- the person on whose behalf an agent acts
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

See a comprehensive dictionary for specialized senses in finance, architecture, law, chemistry, and music.