Ethically, you shall pay something to exchange for other things. However, there is a special thing that should be shared to all others who are limited for the access is knowledge! Due to the fact that, what makes the human is different from other species is the accumulation of knowledge via books, hand-writing and recorded information throughout history [this statement was derived from Scientists!] In the era of data science, Springer offers a hundred of books in free of charge via this link. Enjoy!