What does "letter" mean in this context?

In this context the "letter" of the constitution is being contrasted with the "spirit" (what this author calls "the principles") of the constitution.

It means something like "read literally."

Does that make sense to you? If not I could say more.

Yes, it makes sense. Thank you. So it can mean "literal reading"?

Yes. Which can differ from a reasonable reading.

For example: A statute may say that any vehicle exceeding the posted speed limit is guilty of a violation. That is the literal reading (what the words actually say).

But we don't give traffic tickets to ambulances, or to cops who are attempting to catch up with a criminal.

The have violated the "letter" of the statute, but not it's implicit exceptions. They have not violated the "spirit" of the law.

Likewise, the commandment saying "Thou shalt not kill" wasn't intended to mean that you can't shoot a rattlesnake that is about to strike you, or a duck that you need to eat for your next meal.

I know what literal means. I didn't know what letter meant.

Thanks for the clarification.

You're quite welcome, Perry.