This is a WEB Scraping & WEB Crawler function.
This short function downloads all the PDF book files and saves locally, from the main WEB page, one file at a time, the PDFs are at second level WEB pages for each book:
getBooks[bookUrl_]:=
URLDownload[
Select[Flatten[
Import[#, "Hyperlinks"] & /@
Import[bookURL, "Hyperlinks"]],
StringContainsQ[#, ".pdf"] &],
"~/Downloads/Books",
CreateIntermediateDirectories -> True];
getBooks["https://books.goalkicker.com/"]