I've been using Overdrive for a year now and saved a fortune on Amazon. If you have access to a library in Singapore (via PR, EP library card etc) you can sign up for Overdrive and access the NLB's massive ebook collection.
1. Install overdrive on your phone
2. Search for and then add the Singapore National Library in the overdrive app as a library to borrow from (you'll need your NLB ID and password).
3. Once you have added the NLB to overdrive as a 'library' you can then search in the NLB catalog and download books to your phone. There is a generous limit on the number of books but I find the time duration a bit short (14 days I think).