The official source code repository for the calibre ebook manager
Sure thing - another must-have resource if you get a kindle is good library management software. Assuming you download books instead of purchasing them directly from Amazon. The gold standard is Calibre eBook manager (windows and Mac, 100% free). Using it, you can sideload books to your device using a cable or email.
The source code for the Standard Ebooks website. (by standardebooks)
I use my kindle for reading all of the free classics on Project Gutenberg and Standard EBooks. I also buy the occasional short story collection on Amazon and use my kindle for those. I always have a physical book, kindle book and audiobook going all at the same time. Physical book is main book and for heavier or more serious reads whereas audiobooks are my popcorn books and fun stuff. Kindle is everything in between - use it for travelling and reading in the dark (putting kids to bed and reading in bed next to wife).
The modern API for authentication & user identity. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.