‘Apropos’ means ‘suitable’ or ‘relevant’. Apropos is a French word. ‘a’ means ‘to’ and ‘propos’ means ‘to plan’. ‘Apropos’ means ‘to the purpose;, ‘fitly’, ‘opportunely’.
‘For Example:

The remark was 'apropos’ means that it was a very relevant remark. It was a remark that suited the place and the occasion.
He never says anything apropos.
Is there any passage apropos this? 

It is used to introduce a new topic connected with the earlier topic. You say you met Ram yesterday. Apropos, did you mention our joint project to him?

‘Apropos of’ means ‘with regard to’ or ‘concerning’, ‘in connection with’.
Apropos of the venue of the meeting, I am sorry I cannot tell you anything about it now.
Apropos of the Minister’s arrival, nobody seems to know anything. 
 
You must always remember that ‘apropos’ and ‘apropos of’ have different meanings