For a system of this age (i.e. Workbench 1.3 and 1 MB RAM ) I would suggest WordPerfect for Amiga.
It's essentially a very good text editor (i.e. there are no customizable fonts).....but you don't need that for writing a novel. It has a nice quick spell checker and can run without a hard drive.
There are other wordprocessors that will work: Kindwords, Excellence, Finalwriter, but these are all WYSIWYG wordprocessors, with fancy GUIs that will only slow your system down.
There was a copy of Wordperfect for Amiga on Ebay last week.
I think 4.1.12 was the latest version.