Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose to Repair Database. If that doesn't help, then try again, this time using Rebuild Database.
If that fails:
Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
Thanks for your helpful suggestions, TD. Rebuilding the library (using iPhoto Library Manager) did result in making the jpegs editable in iPhoto. However, it had the unwanted effect of converting the RAW files into jpegs. So I still wasn't happy.
I've made three attempts at restoring the relevant library's image from the Time Machine backups saved on different days when I felt sure I had successfully viewed and edited the images. Those restored images had the same problem = they too were uneditable.
On the other hand, newly imported images (both jpeg and RAW, both new and previously imported) are editable.
Have you (or anyone else) any further suggestions?
1) iPhoto identifies them as JPEGs (image info), whereas it identifies earlier, unaffected RAW files as "RAW"
2) the same with Aperture
3) Photoshop and Photoshop Elements open them and identify them as JPEGs, whereas they open earlier, unaffected RAW files with the Camera Raw plug-in instead.
I can export the "original" files, selecting "original" in the export dialog box, but the exported files turn out to be JPEGs, not RAW files (by the criteria above).
Also, the log file resulting from rebuilding the library with iPhoto Library Manager indicates 360-some images were missing, which roughly corresponds to the problem segment of my library.
These images definitely weren't missing two weeks ago, but when and where they went, I've no idea.