PDF Drive is a free search engine for PDF files that claims to have over 48 million eBooks for you to download for free. No ads, no download limits. Also available is a PDF Drive Premium for $3.99/month or $29.99/year. Cancel anytime. As near as I can tell the advantages are faster download and they can be stored on a cloud so you can access them anytime.
PDF Drive is a free search engine that allows you to search, preview and download millions of PDF files into your devices. Their crawlers are constantly scanning the web to add PDF files to their database. In the case the PDF files are withdrawn from the web, then they are also immediately withdrawn from PDF Drive search results. In this way the PDF Drive library stays up-to-date, while continuously growing. In addition to the traditional search engines, PDF Drive has previews, cover photos, advanced filtering and very fast searching.
A quick search of chess books turned up a lot of them, fairly modern stuff, not those in public domain. One blogger claimed to have downloaded over 60 chess books. I didn't download any chess books, but did download a 400-plus page book on Eisenhower and espionage.
My understanding of the legality of this is the pdf books can be downloaded for reading purposes, but you cannot print or distribute the downloaded copy.