Quick question: how would double-entry bookkeeping change the economic landscape in Westeros?

So, before double-entry bookkeeping, what you basically had were lists of goods and sums of money you had, which is good for counting things, but not very good at telling you much about how your business is doing.

Double entry provides a lot more information: it shows the different kinds of things you have (cash on hand vs. goods vs. credits and debts) so that you can examine each category individually, it focuses your attention on building up equity (i.e capital) and allows you to compare your capital to your debts more easily so that you can have a clear impression of how financially secure you are (it also allows for you to see whether you’re in profit or not as opposed to just how much cash you have atm), it allows for much easier detection of error by checking whether the credits and debts add up and allows you to audit a final amount by checking it against the ledger entries, and it allows for comparisons from period-to-period so that you can see if you’re growing or shrinking and where (which makes it easier to invest more in areas of strength, etc.).