These novels are great fun on nitrous oxide? (3-5)

'these novels' is the definition.
(I have seen 'popular novels' mean 'aga sagas' so perhaps 'novels' could also mean 'aga sagas')

'are great fun on nitrous oxide?' is the wordplay.
'are' becomes 'a' (short for 'are', historical unit of measurement).
'great fun' becomes 'agas' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'nitrous oxide?' becomes 'gas' (nitrous oxide is a gas).
'a' put after 'agas' is 'agasa'.
'agasa'+'gas'='AGA-SAGAS'

Can you help me to learn more?