you could always do an S-expression-esque DSL in JSON ;)

  ['book', {'id': '...'},
    ['title', {}, ...],
    ['chapter', {'id': 0},
      ['title', {}, 'Chapter 1'],
      ...
    ],
    ['chapter', {'id': 1},
      ...
    ],
  ]

more realistically, you could just represent it with the AST of that XML, i.e

  {
    'type': 'book',
    'attrs': {'id': ...},
    'children': [
      {
        'type': 'title',
        'children': ['Simple book']
      },
      {
        'type': 'chapter',
        ...
      },
      {
        'type': 'chapter',
        ...
      },
      ...
    ]
  }

so you could do that emphasis bit as

  [
    'this text nees more', 
    {'type': 'emphasis', 
     'children': ['emotion']},
    '!'
  ]

hellish to write by hand but probably okay for a program to consume (modulo all the XML libs/tooling you can't use). and you could probably even write some kind of schema for it.

if i actually had to represent that data, i'd also move some child nodes into attributes, e.g. make all nodes with 'type': 'book' also have a 'title' attribute, like you would if you had an AST datatype