There isn't really a formal manual, because there's no single style or standard.
So long as you understand the rules of identifier naming you can use whatever you like.
In practice, I find it easier to use lower_case_underscore_separated_identifiers because it isn't necessary to "Double Quote" them everywhere to preserve case, spaces, etc.
If you wanted to name your tables and functions "@MyA??! ""betty"" Shard$42" you'd be free to do that, though it'd be pain to type everywhere.
The main things to understand are:
Unless double-quoted, identifiers are case-folded to lower-case, so MyTable, MYTABLE and mytable are all the same thing, but "MYTABLE" and "MyTable" are different;
Unless double-quoted:
SQL identifiers and key words must begin with a letter (a-z, but also letters with diacritical marks and non-Latin letters) or an underscore (_). Subsequent characters in an identifier or key word can be letters, underscores, digits (0-9), or dollar signs ($).
You must double-quote keywords if you wish to use them as identifiers.
In practice I strongly recommend that you do not use keywords as identifiers. At least avoid reserved words. Just because you can name a table "with" doesn't mean you should.