I agonised on am pm for ages and found FOUR official "styles"
- Caps 7 AM
- small caps 7 AM (fake attempted here fails!)
- dots 7 p.m.
- plain 7 pm
The New Penguin Dictionary of Abbreviations by Rosalind Fergusson was no help. So I decided:
Has to have space between number and am or pm
Whatever you do, do the same everywhere
I decided that (4) was the most common and easy to search and fix to.
I've also decided that
SMALL CAPS are more common in older books and awkward, because in line character styles are more awkward than paragraph styles on conversion. Apart from the ebook display issues.
I listed the 4 options above and my choice on the flyleaf.