Sorry you are right about the cheaper ones listed being owners manuals. I have during the past year seen many "service manuals" listed for the classic Prius. They have all been for about a hundred dollars.
The 2005 set now listed for $128.00 has been selling for about $250.00 and up. They list for $407.00 and are as poorly written as the owners manual. For instance, the Brake system is scattered through out the three volume set and as poor an index as that of the owners manual. There have been no other books "clymer, Haynes, ect;" published for the Prius, probably as the publishers have not been able to decipher the factory service manuals. I have a three volume set of factory manuals for the 2004 and they are the poorest auto service manuals I have ever seen and almost worthless.
There is a CD for the complete manual but it comes with no index.