Hi!
This is mainly a cross-link to a thread in Sony Reader's Dev Corner - but may be of interest to some.
I just made up a little
"DictioLauncher 0.5". This is how it works:
– Dictionaries can be accessed without closing your current book: by inserting the SD card the launcher just "appears" and will "disappear" after use.
– Your dictionary must be a simple text file: no html, no mobipocket or the like!!!

You can search for any word using the onscreen keyboard (use buttons 0 to 9).

Get at different keyboard rows with "cursor-up/down".
. . . (It's enough to type the first letters of a word: the whole word will be displayed...)

The "arrow" (right side) removes letters from your input word one-by-one.

DictioLauncher will check your input word against the first word of each line of your text file.

Search time varies from zero ("a...") to"quite some time" (> 3 s for "z..." in a 12,000-lines-file).

The previous/next line (= word) will be displayed by pressing <page up/down> or <cursor left/right> (search restarted forward/backward).
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– Included are three example dictionary files (French, Italian, Spanish; from
www.freedict.com): Rename any of these to "dictionary.txt" and put them into "Sony Reader/software" on your SD card.
– BUT - you may use *ANY* text file you have/find (thesaurus, to-do-list, etc.).
– Go and find dictionary files for any language you need!
– Your text file should be saved as UTF-8. Sony PRS-500/5 seems to get a "hangover" if confronted with characters like á, è, ß or even ö (o-umlaut)...
(Also the eReader does *not* like "-, *, /" at the beginning of a line while searching your input .)
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SORRY! 
...while editing this post (the URL for the original post) some of it miraculously
got lost... ...and found again.
Here is the link for the dictioLauncher thread:
https://www.mobileread.com/forums/showthread.php?t=22320