A I haven't received the book I ordered.
B I haven't received the book I have ordered.
Which one would you say is more often used / colloquial, and which one is more formal / more grammatically conservative?
A I haven't received the book I ordered.
B I haven't received the book I have ordered.
Which one would you say is more often used / colloquial, and which one is more formal / more grammatically conservative?
The first would be more colloquial, more often used, and more formal and grammatically conservative.
They both use the perfect in the negative for the thing that could have, but failed to, happen at some undefined time in the past, up to the present.
The first uses the simple past for the ordering, the latter the perfect, but since the ordering was a single act that happened at a particular time, and since defining that time defines the period during which the book could have arrived, the simple past would be more normal.
The second remains valid, and could be used correctly in all registers. The first is just more normal in all registers because the choice of tense better expresses the sequence of events.