The Gregory book is not semi-active at all. The 4th edition was a re-write of the 3rd, from scratch, as the original template files were lost. The 4th edition wasn't even updated by the original author, it was done by Marv Shaner, with Gregory's approval. From what I understand there were a few edits made and the registry info updated. I seriously doubt we will ever see another edition. The hi-po website has been hanging on a string the past couple of years, hardly any participation there now.
The Mannel book was a solid resource when it was published, one of the best I have ever seen. I personally would not compare the 2 books as one was done right and the other one more like a pocket reference with a registry attached.
In no way was this intended to criticize but more to point out that some sources are reliable and some are not. If you'd like to wave around the hi-po book as the ultimate reference, then go ahead, but it would be silly for you not to be apprehensive about the content given that there are known errors.