[gluggy]

Here's a little Java program I wrote, which lets you edit, add, and remove EXTH tags in mobi files. It doesn't have as many features as mobi2mobi, but it suits my needs.

I've only tested it with a couple of non-DRMed files, and it seems to work fine.

It's a standalone program, and doesn't depend on any other programs or libraries (apart from Java, of course).

Enjoy!

Update:
v0.14
- Added Open and Save menu items.
- Lets the user specify the target filename.

v0.13
- Added a Header Info dialog, which details various fields in the PDB header, PalmDOC header, and MOBI header.
- Added in provisions to invoke debug and safe modes from the command line.
- Added in WhisperPrep, which lets users set ASINs and sets the CDEType to EBOK
(to invoke, type "java -cp MobiMetaEditorV0.13.jar cli.WhisperPrep
<input directory> <output directory>").

v0.12
- Added in various OS X-specific features.
- Allow users to specify an input filename on the command line.

v0.11
- Added in language features.