Cynbar's Fantastic Technology: Droids

Late-run book is actually very useful.
It reprints almost every droid from all previous books, plus many more.
Includes background on Droids in society, laws concerning droids, and of course, stats for the droids.
If you have droid characters, or someone who wants to use droids, this is the book you really need to suppliment what little is in the core rules.
Each entry also has "performance reviews" from the intergalactic computer holographic bulletin board system with suggestions and munchkin ideas that are pretty fun(ny).

Essential? have to say Yes
Usefullness? Extremely
Availability? yes, but probablly expensive
Liabilities: only includes droids from classic trilogy onwards--no prequel droid info. Many of the droids are from other materials, but at least this time they're handy in one place
D20 conversion: Most of these have probablly already been converted by the new books anyway, but the background information might still be useful.

 

Cracken's Rebel Field Guide
A neat collection of modifications to existing gear and vehicles for those who want to customize. Also contains all of the WEG info on cyberwear/cybernetic prosthetics. (also, how to defeat a magnetically sealed door with common debris that could be found in most trash compactors)

Essential? most important book for those who wish to munchkin
Usefullness? very
Availability? check eBay regularly
Liabilities: some of the changes do complicate the system a bit more than is fun, but those same rules help the GM cheat and create very tough opponents.
D20 conversion: Probablly could be done with some difficulty. For the most part, it would be a matter of figuring out the appropriate modifier/cost to accomodate the conversion to different dice/stats and comparing to whatever price/equipment guides list.

 

Death Star Technical Companion
Everything you ever wanted to know about the DeathStar and the people on it.

Essential? no
Usefullness? minimal
Availability? check eBay regularly
Liabilities: the only thing I see that's really moderately useful are the maps of various sub-sections, most of which are available in other publications. Other than that, most of the art is standard, and the material is largely filler.
D20 conversion: I'm sure it has been done already.

 

Galladium's Fantastic Technology
A mixed collection of gear and equipment in a wide variety of forms. Repeats the jury-rigging rules from Cracken's Rebel Field Guide and some of the prosthetics information. Other items (like the droids) are reprinted in other books. Some of the gadgets were originally in other source or adventure guides, but have been moved here for more convenience.
Dozens and dozens of gadgets and gizmos for flavoring an encounter or distracting munchkins with unexpected applications for simple equipment.

Essential? Good to have
Usefullness? moderate
Availability? easy find
Liabilities: Munchkins will find a way to topple a scout walker with 10 meters of synthrope and a tree branch, just imagine what they could do with some of these toys. Selectively decide for yourself what you want to allow and what you don't
D20 conversion: Probablly already been done with various equipment books.