I got enough books in mid-May and knew I was safe. What do you mean?

1, this is a sentence in Ou Yangxun's calligraphy works, the original text is:

In mid-May, I got enough books to know that my body is safe and my strength has not been calmed down. I am eager to know about your rest. Nothing can be said except sadness, which has passed. Ou Yangxun nodded his head.

2. Explain this sentence:

I received your letter in mid-May. I know you are in good health, but I am in poor health and have no strength. .....