If you don’t know, Forgotten Books is a website “discovering and republishing formerly out of print books”.
Translation : It publishes public domain works, scraping books uploaded to Project Gutenberg, and titles posted to Google Books, then republishes them under its own imprint.
These books are marginally readable at best; most have artifacts from the scanning process blurring the text on one page or another. And there are adverts inside of them too.
Half million titles in its catalogue, and nothing worth paying for.
Yes. Forgotten Books is offering a paid membership to read the same ebooks which you can find elsewhere for free. They want $9 a month for access to this rubbish.
My advice : Caveat emptor