Dear Alan,
many thanks for your feedback!
I have checked the files according to your recommendations:
1) the "Summary" section of the iTunes info window tells that all 4 files end with ".m4b"
2) the "Options" section of the iTunes info window tells that all 4 files are set to Media kind = "Audiobook"
3) the "Build Options" settings of ABB are "M4B".
Maybe I was not fully clear in the initial statement. The files ARE listed in the "Books" section of the iTunes library. But this section is again subdivided in "Songs", "Books" and "Audiobooks". The 4 problem files are shown as "Songs". I will mail a screenshot ...
Can "German Umlaut" characters cause this trouble? All 4 files are "Die drei ???", with titles like "128 Schatten über Hollywood".
ADDED by edit:
After reading this post:
viewtopic.php?f=13&t=3559I checked for files in the "Music" section, and (since I keep the .m4a files from the Audio-CD as well) obviously I found the single files from the respective imported CD. There were no "suspicious" files. BUT: When I renamed the original music file (from "128 Schatten über Hollywood" to "128 - Schatten über Hollywood"), and after Quit & Re-Open iTunes, the entries in the "Songs" section were gone.
It seems there is a bug in iTunes that causes trouble when there are files with .m4a and .m4b that have identical names. However, this happens only in some cases which I am unable to specify, because for me only 4 files where affected although I usually keep all originals in addition to the "bookmarkable audiobook" files that ABB produces.