There are many free online identification keys, on this page we list a few, mostly Swedish, sites:
Please visit our Libguide with eReference Works where you will find more books on invertebrates.
Don't forget that many field guides are only available in print. If you can't find what you need in this guide, do a search in the library catalogue. Alternatively, visit the Biology library and browse our shelves where the books are sorted according to organism group.