https://www.reddit.com/r/csharp/comments/yqli7o/how_to_map_this_json_element_to_class_member_is/ivqlwqe/

Program.cs

using System.Text.Json;
using System.Text.Json.Serialization;

string json = """
{
    "ISBN:9780544003415": {
        "bib_key": "ISBN:9780544003415",
        "info_url": "https://openlibrary.org/books/OL26885115M/The_lord_of_the_rings"
    }
}
""";

var options = new JsonSerializerOptions
{
    Converters = { new IsbnConverter() }
};

var result = JsonSerializer.Deserialize<Dictionary<Isbn, YourItemModel>>(json, options);

Console.WriteLine(result[new("ISBN:9780544003415")].info_url);

record YourItemModel(string bib_key, string info_url);

record struct Isbn
{
    public string Number { get; }

    public Isbn(string isbnString)
    {
        var parts = isbnString.Split(':');
        if (parts is [ "ISBN", var number ] && number.Length is 10 or 13)
            Number = number;
        else
            throw new Exception("Incorrect ISBN.");
    }

    public override string ToString() => $"ISBN:{Number}";
}

class IsbnConverter : JsonConverter<Isbn>
{
    public override Isbn ReadAsPropertyName(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => new(reader.GetString());

    public override Isbn Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => throw new NotImplementedException();

    public override void Write(Utf8JsonWriter writer, Isbn value, JsonSerializerOptions options)
        => throw new NotImplementedException();
}