The principle of layered evidence is either correctly applied or violated. You can read and learn from both. Sometimes, authors don’t care if they reference secondary or primary literature. If you want to work correctly, you’ll have to deal with this behavior. If you read something interesting but the facts are not derived from primary literature, you can’t trust those facts. Always – at least at random – check if the primary literature is used correctly. I fell flat on my face after I first discovered how widespread mishandled and misused primary resources are. There are a lot of people who want to sound scientific as a means of marketing.

So, always check the primary literature if there is any. Take texts that cite only secondary literature with a great heap of salt.