You can't charge someone with theft if nothing is stolen!
If I send you a letter, the physical item (i.e. the paper and ink) is effectively a gift to you and it becomes your property. You're free to burn it, bury it, mount it, frame it or do whatever else you like with it. You're also free to give it to, or sell it to, anyone you like. Whatever you do with it, I can't have you charged with theft, criminal damage or anything else because it's your property.
The previous paragraph, however, only applies to the actual (physical) letter. The contents of that letter (i.e. the words contained in it) remain my 'intellectual property' (i.e. I hold the copyright in relation to the content of the letter). So, if you (or someone you've passed the letter on to) publishes the letter, I can sue for breach of copyright. (Alternatively, if I become aware of the intention to publish, before it's actually happened, I can seek a court order barring publication).
Charles sent copies of his journal to about 50 friends. Each one of them will own the piece of paper which they received. So, if one of them gives a copy to the press, no theft has taken place. The court action relates to Charles trying to protect his intellectual property rights and, as such, is a civil action not a criminal one.
Chris