Korean Textbooks by King Ross - Elementary Korean, Continuing Korean, Advanced Korean

Free Korean books you can borrow from Singapore public libraries (NLB).

Digital Korean language textbooks are available for borrowing. 

First, if you do not already have one, is to sign up for Singapore NLB MyLibrary ID,
https://account.nlb.gov.sg/

Then you can follow the guide for those new to NLB e-books,
https://mobileapp.nlb.gov.sg/get-started-with/libby/

I assumed that ebooks are available in unlimited "copies" but no, so if a popular book is not available, you need to reserve via "Place Hold". 

Of course, one of the first things I did was to search for "Korean" and filter by choosing Subjects -> Language Arts. And I am glad a number of books I often recommend are available as NLB's ebooks and I share them below. I like the series of three books by King Ross and his co-authors:

ELEMENTARY KOREAN

CONTINUING KOREAN

ADVANCED KOREAN