Is there any effort to modularize Backbone itself, so that Bookshelf can only depend on the model and collection pieces?
It's definitely something I'll begin to look into a bit, possibly splitting up the Backbone components into a separate library specifically for Bookshelf, similar to lodash's custom build process. If you have any ideas for a good way to go about this, feel free to open a ticket.
It also has support for handling database transactions, which seemed to be missing in any other libraries I had looked at, and also doesn't force you into a particular way of doing other things like validations or typecasting (though it does provide hooks/events for handling both).
The ORM is also just a single ~900 line file that takes a lot of inspiration from Backbone as being a library you can easily read through and understand, and focus on the important parts of database interaction, rather than trying to provide the kitchen sink.
[0] https://twitter.com/laravelphp/status/333971780886728705
I owe a lot of thanks to Taylor for writing Eloquent, he has been really helpful with any questions I've had in putting this together.
Nice work!