- Developer Guide for Dynamics 365 Customer Engagement (still possible to export as PDF) – definitely take a look at Xrm.WebApi (Client API reference)
- Dynamics 365 assemblies and some tools (e.g. solutionpackager, package deployer and plug-in registration tools) are now all available from NUGET – check out Microsoft.CrmSdk.CoreAssemblies on Nuget
- The Dynamics 365 Entity Reference is also available (and updated) online
For more details check out What’s new for Customer Engagement developer documentation in version 9.0.