The developer guide for lightning web components is located in the component library. There is a public URL for this here:
In addition to the developer guide, new LWC features in the component library also include a new JS playground tool for prototyping LWC code and a resources tab to quickly take you to common resources.
There is currently no plan to house the LWC developer guide in the docs portal going forward.
You no longer need to be logged into a pre-release org to access the component library. The component library public URL now supports direct access to the lightning web component developer guide along with the new playground tool.
!!!!! Spring 19 Pre-Release Note !!!!!
During the pre-release period in order to see the LWC features of the component library, you must access the relative URL from an authenticated org session of a Spring 19 pre-release org. If you go directly to the public URL, you will see the component library of the current GA release, Winter 19:
Simply log into your pre-release org, then append the following onto the host-name of your org:
Once Spring 19 goes GA, in February, the LWC features will become available in all instances when accessing the component library.