A good starting point is the happstack-lite tutorial. It's less than 2000 words long, yet contains everything you need to create a full-featured website:
The Happstack Book (previously know as The Happstack Crash Course), weighing in at over 50,000 words offers a comprehensive introduction to Happstack and many of its optional components:
http://happstack.com/docs/crashcourse/index.html
Happstack also offers comprehensive and detailed API documentation:
happstack-server - HTTP server, routing combinators, blaze-html templating, and more.
happstack-server-tls - extend happstack-server with SSL support
happstack-lite - a easy way to get started with Happstack
happstack-hsp - literal XML/HTML syntax embedded in Haskell
hsx-jmacro - easily embed Javascript into Haskell
happstack-heist - XML templating macro language
happstack-hamlet - HTML templates via QuasiQuotation
happstack-hstringtemplate - string based templating solution
happstack-jmacro - Additional support for using JMacro with Happstack
happstack-yui - Bundles YUI 3.5.1 and includes a "combo handler" for use in Happstack which concatenates YUI modules server-side to send in a single HTTP request.
reform:Type-Safe form generation and validationCovered in this section of the Happstack Crash Course.
reform - core reform library
reform-blaze - support for using reform with blaze-html
reform-happstack - support for using reform with Happstack.
reform-hsp - support for using reform with HSP
acid-state - Power of ACID brought to pure Haskell
safecopy - binary serialization with version migration
ixset - a multi indexed collection type (similar to a SQL table)
web-routes - type-safe URLs and routing
web-routes-boomerang - expressive combinators for defining routes
boomerang - library which contains combinators for use with web-routes-boomerang
web-routes-happstack - use web-routes with Happstack
web-routes-hsp - use web-routes with HSP templates
web-routes-th - use Template Haskell to automatically derive rendering from a URL type
happstack-clientsession - store session data in client-side cookies