Lessons learned.
Meaning: This means that the person reading the book has gained some new knowledge or understanding from it. It's like saying, The reader has acquired some new information or skills.
Choice of Words: Lessons are things that are taught or learned, especially moral or personal ones. Learned is the past tense of the verb learn, which means to gain or acquire knowledge of or skill in (something) by study, experience, or being taught.
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