Select combobox in MVC .net core linked to a ID

I trying to learm MVC .net core and I need to populate a combobox and save in to Database the ID of combobox.

I have this code in my controller:

ViewBag.Categories = new List<SelectListItem>
            {
                new SelectListItem{ Text="history", Value="1"},
                new SelectListItem{ Text="literature", Value="2"},
            };

here the code in my view:

 <div class="form-group row">
        <div class="col-3">
            <label asp-for="IdCategory">Category</label>
        </div>

        <div class="col-6">
            <select asp-for="IdCategory" name="CategoryId" asp-items="ViewBag.Categories">
                <option>-- select the category --</option>
            </select>
            <span asp-validation-for="IdCategory" class="text-danger"></span>
        </div>
    </div>

The combobox show correctly the content but after I save in to the database the field "IdCategory" = 0.

My impression is that the view try to save "history" in field "IdCategory" and not "1" that is the ID.

If this is true, how I can force to save the ID and not the text ?

Thanks

Update here the controller:

public IActionResult Upsert(int? id)
{

    ViewBag.Categories = new List<SelectListItem>
    {
        new SelectListItem{ Text="history", Value="1"},
        new SelectListItem{ Text="literature", Value="2"},
    };


    Book = new Book();
    if (id == null)
    {
        //create
        return View(Book);
    }
    //update
    Book = _db.Books.FirstOrDefault(u => u.Id == id);
    if (Book == null)
    {
        return NotFound();
    }
    return View(Book);
}

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Upsert()
{

    if (ModelState.IsValid)
    {
        if (Book.Id == 0)
        {
            //create
            _db.Books.Add(Book);
        }
        else
        {
            _db.Books.Update(Book);
        }
        _db.SaveChanges();
        return RedirectToAction("Index");
    }
    return View(Book);
}

here the complete view:

@model BookListMVC.Models.Book

<br />
<h2 class="text-info">@(Model.Id!=0 ? "Edit" : "Create") Book</h2>
<br />

<div class="border container" style="padding:30px;">
    <form method="post">
        @if (Model.Id != 0)
        {
    <input type="hidden" asp-for="Id" />}
        <div class="text-danger" asp-validation-summary="ModelOnly"></div>
        <div class="form-group row">
            <div class="col-3">
                <label asp-for="Name"></label>
            </div>
            <div class="col-6">
                <input asp-for="Name" class="form-control" />
                <span asp-validation-for="Name" class="text-danger"></span>
            </div>
        </div>
        <div class="form-group row">
            <div class="col-3">
                <label asp-for="Author"></label>
            </div>
            <div class="col-6">
                <input asp-for="Author" class="form-control" />
                <span asp-validation-for="Author" class="text-danger"></span>
            </div>
        </div>
        <div class="form-group row">
            <div class="col-3">
                <label asp-for="ISBN"></label>
            </div>
            <div class="col-6">
                <input asp-for="ISBN" class="form-control" />
                <span asp-validation-for="ISBN" class="text-danger"></span>
            </div>
        </div>


        <div class="form-group row">
            <div class="col-3">
                <label asp-for="ISBN"></label>
            </div>
            <div class="col-6">
                <input asp-for="ISBN" class="form-control" />
                <span asp-validation-for="ISBN" class="text-danger"></span>
            </div>
        </div>


        <div class="form-group row">
            <div class="col-3">
                <label asp-for="IdCategory">Category</label>
            </div>

            <div class="col-6">
                <select asp-for="IdCategory" name="CategoryId" asp-items="@ViewBag.Categories">
                    <option>-- select the category --</option>
                </select>
                <span asp-validation-for="IdCategory" class="text-danger"></span>
            </div>
        </div>



        <div class="form-group row">
            <div class="col-3 offset-3">
                <button type="submit" class="btn btn-primary form-control">
                    @(Model.Id != 0 ? "Update" : "Create")
                </button>
            </div>
            <div class="col-3">
                <a asp-action="Index" class="btn btn-success form-control">Back to List</a>
            </div>
        </div>
    </form>
</div>

@section Scripts{
    <partial name="_ValidationScriptsPartial" />
}

and the book class:

public class Book
{
    [Key]
    public int Id { get; set; }
    [Required]
    public string Name { get; set; }
    public string Author { get; set; }
    public string ISBN { get; set; }
    public int IdCategory { get; set; }
}