What is the ruling on the action of a former employee who exploits a client list of a bookstore where he used to work for his own personal benefit, and is his action considered a breach of trust and unlawful theft?

The previous employee's dealings with your library's clients—or with clients who came to him through groups he formed—do not constitute a breach of trust, especially if he only used his personal phone during his work with you with your permission, and if you did not stipulate that he should not deal with your clients after he left your employment, and he did not deal with his clients as if he were your employee, but rather on his own behalf or for another library. Therefore, there is no blame on him for that.