So, Charles bored his bride to the point that she had to seek the company of the staff. A 32-year-old man, on his honeymoon with the most famous, sought after young woman in the world, wants to read rather than, oh, take a bubble bath together, feed each other grapes, cuddle, make love, etc. No, he wanted to read a book.
I don't think Charles necessarily "bored his bride to the point that she had to seek the company of staff" nor do I think Diana "abandoned" her husband by not wanting to read with him.
I haven't read the book or the letter(s) in which this story is recounted, so I can't comment on the exact circumstances of the event, beyond the fact that at some point in his honeymoon, Charles wanted to read, asked Diana if she wanted to join him, she declined and let him do so while finding another form of entertainment.
A few points:
1. Some people enjoy reading, some people don't. Neither one is necessarily right or wrong in their behaviour. What's more is that people who enjoy reading enjoy reading different things, and sometimes people who do like to read don't feel like reading.
2. A mature couple should not be expected to share every hobby, like, or dislike. What's more is that they should not be expected to spend every moment together, even if they're on their honeymoon.
Now not knowing the circumstances I don't want to nitpick things too much, but it seems to me that if Charles was insulted that Diana didn't want to read with him he should have grown up a bit. If he wanted to do something with Diana at that moment and she didn't want to do what he wanted, he could have easily suggested something else or asked her what she wanted to do. Likewise, if Diana wanted to do something with Charles at that moment she could have suggested something else.
I don't think it's a problem that at some point they weren't together on their honeymoon, or that Charles liked to read while Diana may not have (or may have preferred a different type of literature). I do see a problem in that Diana is typically described as having misrepresented her interests prior to her marriage. If she told Charles during the courtship how she loved reading Kant, then waited until after the wedding to tell him how she can't stand German philosophy then she misrepresented herself and tricked him.
In Diana's defense though, I don't think Charles was the only one being tricked during the courtship. Diana does seem to have portrayed herself as having been someone she wasn't, but Charles' intentions in the marriage were also misrepresented. I tend to believe that he went into the marriage intending on being a faithful husband, but I do think Diana was lead to believe that this was a fairy tale marriage - that Charles was her Prince Charming who would save her from her life - while Charles was viewing this as more of an arranged marriage - he was obligated to marry and Diana was the type of girl he had to marry. I don't believe Charles intentionally tricked Diana in this regards - I do think her family did so intentionally, and it's likely that people involved with the BRF did so as well - but she was tricked nonetheless - and she let herself be tricked, turning a blind eye to the signs that her reality was not what she thought. Despite that though, I think Charles should be held more accountable in this regards than Diana. Diana was practically a child, and he should have made it clear to her that theirs wasn't a love match or a fairy tale before marrying her.