Commercial confidentiality is often cited as a legal obstacle to greater transparency, but legally it is an obstacle that is readily overcome: it is contractual, not statutory, and the imperative for confidentiality is commercial, not legal. But when businesses engage with individuals – as employees, customers, contractors, suppliers or shareholders – there are real legal obstacles to total transparency. Personally-identifiable information is subject to the Data Protection Act (in the UK, equivalent legislation throughout the EU and other privacy laws in the USA) and much of it is sensitive and must, therefore, be handled securely. Salaries are conventionally considered confidential, though there is an argument that greater transparency would help reduce the salary disparities that seem to be correlated to differences on which it is unlawful to discriminate. Yet even if salaries were transparent (which is a separate issue), there are plenty of other personally-connected transactions the details of which are properly of concern only to the parties involved: phone bills, bank accounts etc.
This, however, is not an insurmountable obstacle either, but it is an additional challenge to the practical implementation of open-book accounting. There are techniques, and accounting practices can be devised, which will protect the sensitive detail whilst revealing the aggregate.