Track Concatenator joins iTunes tracks into single, chapterized MP4 files. Allows output as Music or Audiobook. Requires FFMPEG/MP4v2 (can be installed on first run).
Joins MP4/AAC files losslessly without reencoding; joins other filetypes (or mixtures of filetypes) with reencoding to MP4/AAC at the highest bitrate of the input files.
See demo video: How to merge tracks in iTunes.