Books
Books are a concept, a collection format, a way of organizing ideas; including ebooks, textbooks, picturebooks and others. There are many sources of Free books in the public domain, or under GDL, various Creative Commons, and other licenses.
Ebooks
An Ebook is simply a book that is stored in a computer and read on a computer screen. We simply talk about digital books as "books" on the wiki.
It is entirely data and does not include any application. This last point is stretched somewhat by recent versions of Adobe Acrobat's PDF reader which can handle embedded Javascript applications in a PDF document and for HTML files containing Javascript. But for the purposes of this page we should restrict ourselves to the simpler, pure data digital books. There will be a place for rich documents containing Javascript, but these are really applications with lots of data, not books as such.
The OLPC literature page links to many sites offering collections of free (Public Domain out-of-copyright) and commercial Ebooks in many languages. There are also specific Ebook collections being developed by libraries and archives for OLPC (add your own!).
Ebook Formats
Book formats should be compressed (to conserve space) and open. In particular, they must not be encumbered by patents, and must be inclusive - they should not favor any particular vendor.
DejaVU
The DJVU format was developed in order to provide a much higher level of compression for scanned paper books, than existing formats like JPEG and TIFF can provide.
The PDF format is a simplified form of the Postscript programming language that only includes the commands necessary to paint ink on the page. It is easy for end users to create PDFs with the Print function of a word processing or drawing application. There are extensive Free/Open Source libraries of functions for creating, editing, and otherwise modifying PDFS, and applications built from them. For example, libpoppler and the Poppler PDF Utilities. There are also several Free PDF display programs, including xpdf, kpdf, evince, gv, and ViewPDF.
HTML
Although not particularly designed as a book format, HTML is widely used for books. Most newer Project Gutenberg books are available as HTML. Both special purpose Ebook readers and web browsers can be used to access HTML Ebooks. The OLPC does include a web browser.
XML
XML is not a directly usable format, but rather a meta-format. XML alone is not a book format, but many modern formats that can be used for books are XML based, such as ODF, and the XHTML variant of HTML. Other XML based formats are DocBook, popular for computer manuals, or TEI, used in the Humanities. Modern web browsers can render XML directly, but to make such a display attractive some transform (expressed in CSS or XSLT) may be required.
OpenDocument
OpenDocument is a compressed format (zip-compressed XML) for documents, including books, presentations, and spreadsheets. Complex documents (with many images) can be sent as a single document (unlike HTML), yet it can flow in a display (unlike PDF). It is also editable. AbiWord runs on the OLPC and is capable of opening ODF files.
Greenstone
Greenstone is a monolithic format for document collections. A Greenstone library allows quick full-text search access to large collections, and is typically smaller than the full-text it contains, due to the compression scheme it uses. A Greenstone library can be both accessed via a web server or locally on a (read-only) disk. A complete Greenstone collection can be large, which makes it less useful, given the storage constraints of the OLPC.
Book Readers
Evince
The Evince book reader is part of the OLPC project. Currently it supports DJVU, PDF, Postscript and DVI. The OLPC project will likely include only DJVU and PDF as well as an XML/HTML based format. It will also likely have a modified UI targetted to kids.
Plucker
A popular book format for PalmOS devices. A reader for Linux/X11/GTK+ already exists in the plkr.org CVS codebase (and is distributed in the Plucker 1.8 source tarfile). It should run on OLPC machine's OS. Most Project Gutenberg books are already available in Plucker format at the URL
http://www.gutenberg.org/cache/plucker/NUMBER/NUMBER
where NUMBER is the PG book number for that book. For instance, John Stuart Mill's autobiography is available from PG in Plucker format as http://www.gutenberg.org/cache/plucker/10378/10378.
FBReader
An open source Ebook reader.
OpenBerg
OpenBerg Reader is a multiformat book reader based on Mozilla technologies. It's far from complete but it can already display rich XML/HTML books and organize libraries. Livesearch is in the works, as well as Plucker compatibility. The project could use help.
OpenDocument Viewer
The ODF Fellowship's OpenDocument Viewer could be used as a reader for OpenDocument files. It is open source.
OLPC Features
There are some pages already discussing how book formats and book readers could be specially adapted to the OLPC: