This isn’t a complete list but these are good places to start your search for full-text digital books – but don’t forget your local libraries and librarians.
First, sometimes you need good bibliographic info (e.g. correct spelling of author name, exact title, etc.) before you begin your search for the full-text. Worldcat dot org is a good catalog to find that info. Your own Oregon public library may have a full-service subscription version you can use.
Second: General web search engines will index the contents of many of these repositories, but not all content.