* Then read this, all of it, except perhaps most of the exhibits: Most recent Costco annual report (10-k).
* Berkshire Hathaway annual letters, Transcripts from Berkshire annual meetings
* Great follow-up: Financial Shenanigans, Fourth Edition: How to Detect Accounting Gimmicks and Fraud in Financial Reports (Schilit, Howard, Perler, Jeremy, Engelhart, Yoni, McGraw-Hill Education)
* Absolute classic (but needs to be updated): The Intelligent Investor: The Definitive Book on Value Investing. (Graham)
* On investing in general: Margin of Safety (Seth Klarman)
* Great intro to valuation, with the caveat that modern finance and concepts like WACC may not necessarily be useful to your investing strategy: Valuation: Measuring and Managing the Value of Companies (Tim Koller, Marc Goedhart, David Wessels)
* Another classic (but needs to be updated): Security Analysis (Graham and Dodd)
I've read those in more-or-less that order, interspersed with plenty of other useful books that you'll discover depending how deep you go. The most important thing is reading 10-ks.