m4b-tool list (list commands)
m4b-tool merge --help

Here is the command line I use in Terminal to create a single m4b audiobook with chapters from mp3 files in a directory. The mp3 files names will be how the chapters will be shown in the m4b audiobook. Put cover.jpg in the same directory.

m4b-tool merge -v --artist "God" --name "Bible King James Version" --use-filenames-as-chapters --no-chapter-reindexing --audio-bitrate 32k --audio-samplerate 22050 --audio-codec aac --audio-profile aac_he_v2 --jobs 4 "/home/mint/Music/bible" -o "Bible.m4b"

-v = verbose

--artist "Seneca" = author of audiobook for metadata. Title and Length metadata will be automatically included.

--name "Letters from a Stoic" = title of the audiobook for metadata

--use-filenames-as-chapters = name your files as you want the chapters to be named

--no-chapter-reindexing = forces it to use chapter names especially on large audiobooks

--audio-bitrate 32k = 32kbps seems to be good enough

--audio-codec aac = make sure you compiled or installed ffmpeg with libfdk_aac for best audio quality (very important) as detailed in the installation instructions.

--audio-profile aac_he_v2 = (Advanced Audio Codec High Efficiency version 2) saves 2-7MB or so per audiobook.

--jobs 4 = I have a quad-core CPU so I specify jobs 4 so it uses all CPU cores simultaneously
in quotes "put the path" to the .mp3 chapters to create your audiobook

-o "author and title of m4b audiobook" = output filename (make sure you put the .m4b extension as it will error and won't work)