There are currently two single use(per account) Kobo coupons that can be used on any coupon eligible titles(non agency)
75% off OPEN75UP
90% off DSCVR90BK
These were working as of last night but the expiry date is unknown.
You will get best bang for buck by using it on text books, or fiction bundles(collection,trilogy,omnibus etc)
You may also find it useful to switch regions by modifying your account address some books that are not coupon eligible in Australia may be in South Africa, UK, US etc