Amazon now provides their complete font set on Amazon Alexa Design Guides, the Typography page. There's a big blue Download button on the right of the page, which leads to this link (it's a direct download). The ZIP package contains all Amazon Fonts, including Amazon Ember, Amazon Ember Display and Bookerly (a serif font), with both desktop format (TrueType TTF) and web formats (Embedded OpenType EOT, WOFF and WOFF2) packaged.
A file named
Amazon Ember Licensing Guidelines.pdf is also bundled in the ZIP package with some directives that I don't fully understand. Relevant parts are quoted below (emphasis mine):
Amazon has created a limited library of typefaces for use across all products, interfaces, and websites. These fonts are fully owned by Amazon, making their usage free to all internal product teams. Fonts other than those listed below, such as Palatino, Baskerville, Caecillia, and others are licensed from their respective rights holders (generally Monotype Corporation). To use these other fonts with your product you must secure a license that covers the distribution and display of the desired fonts.
All of these fonts were developed under contract for Amazon - as a result Amazon owns them fully outright. To include these designs in your work there are no licenses payments, no royalties, no copyright or attribution notices required. The font itself carries the proper ownership and copyright information in the header metadata of the font and Amazon does not require any additional attribution or protections when using the font.
The text may suggest that anyone (within or outside Amazon.com, Inc) may use the fonts anywhere but I couldn't secure this assumption, so use at your own discretion.