Using the Bloomberg terminal, you can find ratio metrics with the 'FA' (financial analysis) or the 'RV' (relative value) function. Ratios are available for public companies in the United States and internationally.
Introduction to Business Valuation:
Business Valuation is typical defined as the economic or monetary value of an entire business or company. There are multiple methods for valuing businesses, such as looking at the book value using the balance sheet or multiplying revenues based on industry specific rules of them. This definition from Investopia is a good place to start to get a general overview of business valuation
Example "Rules of Thumb" for Business Valuation:
Books on Business Valuation:
Keyword searching for business valuation or using the subject heading "Business enterprises -- Valuation" will bring up several titles.