The Art of M&A / Due Diligence 
An excerpt from The Art of M&A, Fifth Edition: A Merger, Acquisition, and Buyout Guide by Alexandra Reed Lajoux

The due diligence process begins from the moment a buyer senses a possible acquisition opportunity. The buyer then starts to examine the information that is readily available at this early time about the company. For public companies, this information is usually derived from public documents—including press reports, filings with securities regulators, and any debt or equity offering memorandums the company or its bankers might have prepared for potential buyers.