This is how I combined all mp3s and then converted all of them to m4b:

ls | grep "mp3" | awk '{printf "file |%s|\n", $0}' | sed -e "s/|/\'/g" > list.txt \
  && ffmpeg -f concat -safe 0 -i list.txt -c copy output.mp3 \
  && ffmpeg -i output.mp3 output.m4a \
  && mv output.m4a output.m4b

I found this also to loop through the folder finding mp3s and changing them to m4b:
for file in *.mp3; do name=`echo "$file" | cut -d'.' -f1`; echo "$name"; ffmpeg -i "$file" -b:a 320k "${name}.m4a"; done
for file in *.m4a; do
    mv -- "$file" "${file%.m4a}.m4b"
done
But it doesn´t work and gives me this error:
Could not find tag for codec h264 in stream #0, codec not currently supported in container
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:1 --
[aac @ 0x56453e929e80] Qavg: nan
Conversion failed!