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[Gweilo]

M4B is a kind of MP4 file developed by Apple iTunes; it is an audio track, AAC encoded, with tags like MP3, and also bookmarks and a list of chapters (similar to a .cue file).
Audible's AA/AAX are encrypted M4B and InAudible can decrypt that, so it is becoming a more common audiobook format generally.

Aside from iTunes, Audiobook apps, like Smart on Android, can read and use the chapters.

But if you have a player that plays M4A, but doesn't see chapters in an M4B, or have a huge M4B (40+ hours) that chokes your player, you may want to split the M4B to separate files:

M4acut splits M4B to one M4A for each chapter, lossless.
The releases page: https://github.com/nu774/m4acut/releases
has a zip download with an exe.

The command line is  
m4acut -c file.m4b

to split an m4b to chapters.
You can just make a shortcut that does that when you drop an M4B on it.

And it also can use a separate cue file.
If you have an m4b without a cuesheet, you can make one by using fre:ac (below) to convert the m4B to, e.g. flac, and check the "Create cue sheet" box. After the conversion, delete the flac files and you have a cue file you can use or edit.
You can also see the chapter times using MediaInfo, if you want to manually set cut times.

If you then want to combine some of these chapters into a new M4B, try this method using Chapter and Verse.

fre:ac converts M4B to set of MP3 (or whatever format) -- lossy
Freeware from https://www.freac.org/

Load an M4B and it will parse the chapters as virtual files and from there you can convert to MP3 at whatever settings you like. This is a lossy conversion, as any codec conversion must be.
(Use the same or lower rate than the source, you cannot improve quality by reencoding to a higher rate-- and always use a VBR quality setting, not CBR, which is bigger but not better quality.)
Note: fre:ac can also take a folder of MP3 and compile them into one M4B.

Both these programs use the tag info in the M4B to tag their output files. Pretty good but you can review and change those with MP3tag, which works on both MP3 and M4A, and can edit a folder of files at once.