I just quickly reviewed BestBooks and can immediately tell is it a plugin to be avoided.
Why? To start it created three tables: Accounts, Journal and Ledger. While creating tables is not preferred, the real problem is that it did not prefix table names with $wpdb->prefix which is a best practice in WordPress plugin development.
Secondly IMO it should have used custom post type for Journal and Ledger and it should have used a custom taxonomy for Accounts.
While the latter architecture choice is my opinion, the former issue is definitely a lack of best practices. And what concerns me is if the developer didn't know to use $wpdb->prefix, which is a well known best practice, what other best practices did they miss too, especially security related?