Wonders if one should intercept airmail book when dispatched (9)

'wonders' is the definition.
(mirabilia means wonderful things)

'one should intercept airmail book when dispatched' is the wordplay.
'one' becomes 'i' (Roman numeral).
'should intercept' indicates putting letters inside.
'book' becomes 'b' (abbreviation for book).
'when dispatched' is an anagram indicator.
'airmail'+'b'='airmailb'
'airmailb' anagrammed gives 'mirabila'.
'i' going into 'mirabila' is 'MIRABILIA'.

'if' acts as a link.