Mock IHttpContextAccessor in Unit Tests

You can use the DefaultHttpContext as a backing for the IHttpContextAccessor.HttpContext. Saves you having to set-up too many things

Next you cannot use It.IsAny<string>() as a Returns result. They were meant to be used in the set up expressions alone.

Check the refactor

[Fact]
public async Task test_GetBookByBookId() {
    //Arrange

    //Mock IHttpContextAccessor
    var mockHttpContextAccessor = new Mock<IHttpContextAccessor>();
    var context = new DefaultHttpContext();
    var fakeTenantId = "abcd";
    context.Request.Headers["Tenant-ID"] = fakeTenantId;
    mockHttpContextAccessor.Setup(_ => _.HttpContext).Returns(context);
    //Mock HeaderConfiguration
    var mockHeaderConfiguration = new Mock<IHeaderConfiguration>();
    mockHeaderConfiguration
        .Setup(_ => _.GetTenantId(It.IsAny<IHttpContextAccessor>()))
        .Returns(fakeTenantId);

    var book = new Book(mockHttpContextAccessor.Object, mockHeaderConfiguration.Object);

    var bookId = "100";

    //Act
    var result = await book.GetBookByBookId(bookId);

    //Assert
    result.Should().NotBeNull().And.
        BeOfType<List<BookModel>>();
}

There may also be an issue with the Class Under Test as it is manually initializing the HeaderConfiguration when it should actually be explicitly injected.

public Book(IHeaderConfiguration headerConfiguration, IHttpContextAccessor httpContextAccessor) {
    _httpContextAccessor = httpContextAccessor;
    _tenantID = headerConfiguration.GetTenantId(_httpContextAccessor);
}