For reflashing I would suggest hackers edition 2007...
you could go with the latest 2008...but you may find it has some issues...I have found in the past it had some quirks. But hackers 2008 will have more and greater compat with apps that are in line with diablo/ chinook.
https://web.archive.org/web/20131117.../#sdk_releases

As far as host computers go...
If you are a M$ user...xp works best and least issue free.
win7 can work alright ...installing libusb will most likely be necessary.

"Where can I find a FBReader package that I can install onto the Nokia from my Windows laptop?"
I don't particularly see the need...since you should be able to automatically install all the catalogues necessary for the 770 FROM your 770 via gronmayer...(WARNING! make sure to disable or remove current catalogues from the application manager (under the drop down menu in app. man. under "tools" then in "Application Catalogue") BEFORE adding gronmayers epic 770 list . Otherwise conflicts will occur.)
Then just open your application manager and install fbreader.
OR
From your 770 ...Just go to the download section here at Meamo Central click on os2006 or os2007 or os2008 ...depending on the firmware you are decided on running...and hunt down fbreader there...hit the install arrow

If you wish to have a ton of potential applications to download...

I'd suggest checking out gronmayer's and specifically use the search function for fbreader to narrow the search for the package and make sure it works with your 770. Odds are there are fbreader packages there meant for other os's that may or may not work. some testing may be needed to find the right fbreader packages for you and the os you wish to run whether it is official nokia 2005 - ...or either of the hacker editions...

good luck.