1. IMC'12 paper "Inside Dropbox: Understanding Personal Cloud Storage Services" by Idilio Drago, etc. (Nov 2012).
The paper studies the Dropbox architecture and traffic by intercepting and analyzing traffic data. It provides many insights about various control and data flow, service components, protocols, etc.
2. "Streaming File Synchronization" from Dropbox tech blog (July 2014).
This blog post gives an overview of file sync and presents a new stream-based sync mechanism that improves latency by upto 2X.
3. "Inside LAN Sync" from Dropbox tech blog (Oct 2015).
This blog post describes a new enhancement called LAN sync, which allows devices on the same network to share files without upload/download through Dropbox servers.
1. Core Concepts in dev API doc, especially the Data Model, which defines various data structures, familiar to you if you use Evernote a lot, like me.
2. Synchronization specification in dev doc: "Evernote Synchronization via EDAM" (Jan 2013)
If you are interested, you can also look into FUSE, which is a user space filesystem abstraction. You can build your own Dropbox and Evernote ;-)