Here's a short list of books that are useful for identifying local land snails and slugs.
A field guide to the non-marine molluscs of South Eastern Australia by Brian Smith & Ron Kershaw is the handiest. Or would be were it not out of print. You might be able to get hold of a copy in a second hand book shop or on e-Bay.
Gary Barker's Naturalised terrestrial Stylommatophora in the Fauna of New Zealand series is also very useful, although it obvious covers a subset of species. (Those introduced to New Zealand that have also been introduced here.) There's plenty of technical stuff that might be more than you need but it includes more pictures than you can poke a stick at.
If you're in Queensland, the Queensland Museum's guides to the Wildlife of Greater Brisbane and Wildlife of Tropical North Queensland will help with some of the more frequently-encountered species. The taxonomy is a bit dodgy, though.
But anywhere else in the country and you're in trouble. It's the scientific literature for you!