Aglona Reader is a free open source program for Microsoft Windows + .net Framework for viewing, creating and editing parallel texts in PBO format. The following was taken from the web site with some edits. The Using the Reader section is not on the web site.
PBO, Parallel Book, file format was specifically designed for storing parallel texts used for language learning. Files in this format have PBO extension and essentially are XML files containing texts of an original book ("source") and its translation in another language ("target") as a set of so called "fragment pairs". See PBO for details.
PBS format is the same as PBO with the addition of Sound. It is done by syncing MP3 files on a per page basis.
 Features of Aglona Reader
- Unicode support for all languages with left-to-right script, including all European and East Asian languages like Chinese, Korean or Japanese. Right-to-left languages (Arabic or Hebrew) are currently not supported.
- Smart text formatter can automatically breaking lines when it is required. It ensures that both parts of any pair start on the same row no matter what font, size, or column width you adjust in the settings. At the same time, if line breaks are not required, consecutive fragments are output on the same line, producing an effect of parallel reading of two seamless books.
- Direct use of Windows API functions for rendering and measuring text allows to maintain high rendering fluency even on slow machines.
- Aglona has two modes of work: Reading for viewing PBO files and Editing mode for creating and modifying files.
- Multiple fonts and sizes available. One one at a time.
- Adjustable width ratio of the two columns (applicable to the Normal mode).
- Seamless swap of source and target texts both in Reading and Editing modes.
- Four types of color highlighting of fragments (with optional gradient highlighting of first words and solid color highlighting of whole fragments).
- Three reading modes: Normal (two columns with color highlighting), Alternating (for beginners) and Advanced (showing the text only in one language with the ability to display pop-ups with translations by clicking the right mouse button).
- Adjustable brightness of color highlighting.
- Browsing the TOC, table of contents, of the book.
- Copying texts selected in the Reading mode to clipboard (to select, press and hold Ctrl first).
- Opening a web browser with Google Translate inside the program window and conveniently look up words and phrases in it (click words and select phrases with Ctrl key pressed).
- Storing the list of recent files and their corresponding positions, so that books open exactly where you stopped reading or editing them last time.
 Editing mode Features
Easy and powerful editing mode, allowing to:
- Create a new Parallel Book from scratch by importing two text files in UTF-8 encoding.
- Separate pair fragments from the remaining non-aligned text with as few as two mouse clicks.
- Automatically suggest fragment borders based on line-breaks and punctuation. The user can apply the suggested fragment with SPACE key or adjust it with either mouse or keyboard.
- Edit the text (e. g. correct typos or provide a better translation) in fragment pairs.
- Specify the Structure level and Paragraph starting flags in the Edit pair window.
- Merge any two adjacent pairs.
Support of parallel books with audio (in PBS format) with discrete and continuous playback modes.
 Using the Reader
This section describes the use of the Reader as a reader in Windows.
- The program always starts in full screen mode. You can resize it if you wish.
- The reader has a standard file menu for opening the book of your choice. It will remember the book and where you are reading if you close the program. A list of recent books is also available.
- You can page through the book using the page up and page down keys or click the progress bar above or below its current location. The arrow keys can be used to scroll a line at a time. All other single keys are ignored. Control keys and alt keys can be used as short cuts to commands.
- The TOC is available under Book/Structure command.
- reading progress is shown at the bottom and graphically on the scroll bar.
- The book menu can be used to reverse the book from the source to the translation.
- You can use the Settings menu to set side by side reading (normal), alternate fragments in line, or to select only one language to show (advanced).
- In advanced the book/reverse command will switch languages. The right mouse button will show the fragment pair with a box around each one. Tapping the right mouse again will remove the translation fragment. It is not clear how you would do this with a touch screen. The tab key will toggle between one display and dual display.
- The sync fragments are marked with a color scheme of alternating colors. You can turn this off using the Settings/option command. This command will also let you select the Font to use and size of the font (a scroll bar). There doesn't seem to be a way to view the exact size.
- The Book/find (CTRL-F) command can search for text. It will display the line number of the found data in the lower left corner. You will need to scroll to make it the top line.
- A right click will highlight a fragment. Right click again to remove it. In advanced mode this can be used to show a fragment translation.
- The tab key will highlight matching fragments. Tab again to remove.
- The Book/Information command will shows the metadata such as author, title, other info.
- There is a command to export the book to a single page HTML file using a two column table. Be sure and add the .html extension manually. Fragments are marked by block to remain in sync. If the table were made invisible it could a working starting point for ePub use.
- At one point I found that Clicking text would draw a box around it for some unknown reason. Use the esc key to remove the box. Later this went away.
- It would be nice if the space bar would change pages while reading.
- It would be nice if the page size was remembered for the book, or even just for opening the application.
- There is no clear distinction between the reader and the editor. The menu system is the same. It is easy to change something you didn't intend. Be careful if the program offers to save the file that it is an intended change.
There are sample eBooks available at: Algona reader site