Probably need a Springer account or something along those lines. I'd suggest using scihub (schi-hub (dot) se) to download the books using the links provided in the medium article.
A few months back Springer offered all these books plus a lot more for free download. I'm pretty sure I downloaded all of them. Here is a magnet link if you still want them. It's 15.3Gib.
[+] [-] qorrect|5 years ago|reply
[+] [-] Octokat|5 years ago|reply
[+] [-] skriptmonkey|5 years ago|reply
magnet:?xt=urn:btih:3e2a5671298ce12028899cedcef12c9514a656e4&dn=SpringerBooks