Guidelines to download available public domain texts from Google Books.
Google Books is a web service which provides access to text of books and magazines that Google has scanned, converted to text using optical character recognition. Either "full view" or "snippet view" is available, with some public domain texts free for download.
Wikisource prefers DjVu files with text layer for proofreading. Please try Internet Archive first to locate an already existing DjVu file for adding to Wikisource. This help page focuses on PDF files, the format used in Google Books.
You have to delete the first page from the downloaded file (one with Google logo) since that page is created by Google and does note contain explicit status of its copyright. There were cases when such files are deleted by Wikicommons administrators (e.g. 1).
After locating the file, consider removing the Google page advertisements and watermarks and then upload the file to Internet Archive for DjVu conversion and adding text layer for ease of proofreading. You can ask for help at Help:Internet Archive/Requested uploads.