The book viewer is already built-in (in Newton ROM). See your Newton User Guide for help to turn pages, view TOC, annotate, set bookmarks, etc.
There is a separate application called Bookreader (Bernie Bernstone's reader) that allows you to do that: http://www.unna.org/unna/utilites/BerniesBookReader/
You can also read books using the books routing menu (action button) if you have the HTMList extension installed: http://www.kagi.com/weyer/#HTMList (this also allows you email a specified range of book pages)
With Steve Weyer's PilotDOC plugin for Newt's Cape, you can read PilotDOC books. Please see http://www.kagi.com/weyer/#PilotDoc for more details.
You can use Apple's software to design books from RTF files or any other XTND format (Newton Press) or from files with a special format (with Newton BookMaker).
Links are under Press and Development Software (VB2).
Yes, Steve Weyer's Newt's Cape can create books directly on the Newton from HTML pages: http://www.kagi.com/weyer/#NewtsCape
There is no PDF viewer on Newton yet (and probably there will never be one). However, you can use Adobe PDF-HTML conversion from http://access.adobe.com/, and then view the result into your favorite browser. Besides Newt's Cape by Steve Weyer allow you to save the result as a Book package (this will compress the data).
PDFConv, by Hendrik Lipka and Eric Schneck, can convert a PDF to pictures and a HTML file which you can use with newtscape.