This is the 4761st most frequent German word.
zusätzlicher
"Additional."
'zusätzliche' modifies 'Bücher' as an attributive adjective indicating added items beyond what was already present.
Ich habe einige zusätzliche Bücher für das Studium gekauft.
I bought some additional books for the studies.
'zusätzliche' describes 'Gebühr', expressing that the fee is extra beyond the regular cost.
Es wird eine zusätzliche Gebühr für Expressversand erhoben.
An additional fee is charged for express shipping.
'zusätzlicher' modifies 'Platz', specifying the space that is beyond what was initially available.