Main Site: 
http://nhd.usgs.gov/

Feature codes (FCODE) by layer:
http://nhd.usgs.gov/userguide.html

Standards for National Hydrography Dataset
http://nationalmap.gov/standards/nhdstds.html

Federal Standards and Procedures for the National Watershed BOundary Dataset (WBD)
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_021581.pdf

NRCS Developed .lyr and .style files and instructions for NHD data layers are located at:
https://nrcs.app.box.com/v/gateway/folder/40057621887

Download: 
ftp://rockyftp.cr.usgs.gov/vdelivery/Datasets/Staged/Hydro/State/HighResolution/ (1:24K or better scale)

Model structure: 
http://nhd.usgs.gov/NHDv2.2_poster_062614.pdf

NHD Data Dictionary provides details on all feature classes and attribute tables 
http://nhd.usgs.gov/userguide.html

User Guide: 
http://nhd.usgs.gov/userguide.html

Quick Start - Utility Network Analyst: 
http://nhd.usgs.gov/NHD_Quickstart_Feb_09.pdf

Using the NDHFlowline and NHDFlowlineVAA tables: 
http://megug.org/wp-content/uploads/2010/04/introduction_to_NHD2010_SOM_External.pdf