If you haven't added the Star Fissure symbol to your Mystcraft age, Star Fissures will not generate.
AFAIK, libraries have the same chance of spawning in any and all biomes (excepting void,) though I would recommend a flat biome (e.g. plains, tundra, desert) for spotting them easily.
Once a Descriptive Book has been written, symbols cannot be added or removed from it, whether it is randomly generated or written by you.
My theory is that Rei's minimap bugged out and switched your waypoint to a different dimension by accident upon crashing. I'm not sure where Rei's Minimap keeps its waypoint data, but it will be in a plain text file that you can edit using a program such as Notepad, Notepad++, etc. If you can find where this is, you can change the waypoint that you set to be visible in your Mystcraft age, or copy down the coordinates onto a piece of paper so that you can trek back there via F3. Otherwise, I'd say your best bet is heading back to 0, 0 (a good portion of the time, the spawn point for an age is at 0, 0) and searching in a radius of roughly 100-250 blocks around there to see if you can't find your base.
I found the waypoint lists in "Mod Folder" -> "Minecraft" -> "Mods" -> "rei_minimap", but sadly that waypoint was not in any of my dimensions. It has truly been deleted. :-/
I'm playing on a server so I can't just check the world file. My friends haven't been playing on it much lately but I guess as a last resort I can ask one to come to the world and give me the coordinates.
Edit: hmm, this is strange. Before quitting last time I added another waypoint for the village I was at, yet when I logged in this time it was not loaded. I looked in the file and the waypoint was still there, and when I remade the waypoint the file changed but deleted the old one. Probably the cause of my problem in the first place...
"The mind is the attribute of man. When man is born, he comes into existence with only one weapon with him- The reasoning mind."
...And his fists for punching trees.