Difference between 'with reference to', 'with regard to', 'concerning' and 'apropos of'

Question

1. I spoke few words with reference to this book, 'honesty is the best policy'.
2. I spoke few words with regard to this book, 'honesty is the best policy'.
3. I spoke few words concerning this book, 'honesty is the best policy'.
4. I spoke few words apropos of this book, 'honesty is the best policy'.

Notes for the examples mentioned above:

1. When I say with reference to this book, I mean that the book itself says that honesty is the best policy. [I put a reference here]

2. When I say with regard to this book, I have said about the book that honesty is the best policy. Concerning and apropos of also mean the same as with regard to.

The Oxford English Living Dictionaries defines it in the following way, that shows that apropos of, with reference to, and concerning have the same meaning:

1. with reference to; concerning

I know that there is distinction in preposition choice, as in:

• with reference to . . .
• with regard to . . .
• concerning [zero preposition].
• apropos of . . .

If anyone of you say that with reference to means the same as rest of the prepositions, then please, is it grammatical to use these prepositions in the same sense? Or, if you think that there is difference in the usage, formality and informality, then please make me understand about that.

Besides, is there any connection of 'as concerns' and 'in respect of' with the four prepositions mentioned above.

Answer 1

These words are generally interchangeable and are all formal, according to BBC, though there are some subtle differences you should take note of.

with reference to is, as described by the BBC article linked above:

[You could also add 'with reference to' as] a further alternative and this would perhaps be most formal of all. This expression is frequently used at the beginning of business letters:

'Dear Ms Irvine, With reference to your fax of yesterday, I am pleased to inform you that...'

When it comes to with/in regard to, one person states that:

The phrases using regard tend to refer to an abstract or general object that is the topic or area discussed by the containing sentence or paragraph.

Example: With/In regard to music, he is quite talented.

This claim doesn't have any proof to back it up, so I have my doubts, especially when examples prove the contrary.

One thing that does have proof backing it up, however, is as regards. The word means exactly the same as all four words you mentioned, with a minuscule difference, as pointed out by BBC:

Note that expressions like as far as... is/am/are concerned and as regards link discourse in a similar way, but these are slightly less formal and more characteristic of spoken discourse:

'There is no doubt that in this country infectious diseases such as tuberculosis and diphtheria are on the increase, but as far as whooping cough is concerned / as regards whooping cough, there are clear signs that it is on the wane.'

concerning should be used with utmost caution, since according to Grammarist.com:

The word is also a preposition meaning in reference to or regarding, and the adjectival concerning can cause confusion when readers or listeners initially interpret it as the preposition.

For instance, if you hear someone say, “His email was concerning,” you might at first expect something to come after concerning. This complaint isn’t a rock-solid case, though, as many words in English have multiple functions, but it’s a good reason for those inclined against the word to continue avoiding it.

For apropos, The Guardian style guide has this to say about it:

Used most commonly to mean “with regard to”...

Though I do feel apropos should be avoided just because of how pretentious it sounds.