public class BookContext : DataContext<DbContext>, IDictionaryDbContext
{
    // Из текущего проекта
    public DbSet<Book> Books { get; set; }
    public DbSet<Author> Authors { get; set; }

    // Справочники из My.Dictionary
    public DbSet<BookType> BookTypes { get; set; }
    ...
}

public abstract class BaseController<TContext> : Controller where TContext : DbContext
{
    protected TContext Context { get; set; }
}

public class BookController : BaseController<BookContext>
{
    public BookController(DbContextOptions options)
    {
        Context = new BookContext(options);
    }      
      
    public async Task<IActionResult> Index()
    {
        var repo = new Repositories.BookRepository(Context);                       
        ...
        return View();
    }
}

services.AddDbContext<ApplicationContext>(options =>
    options.UseNpgsql(onfiguration.GetConnectionString("ConnStr")));

var contextOptions = new DbContextOptionsBuilder()
    .UseNpgsql(Configuration.GetConnectionString("ConnStr")).Options;
services.AddSingleton(contextOptions);

services.AddDbContext<ApplicationContext /* ??? */>(options =>
    options.UseNpgsql(onfiguration.GetConnectionString("ConnStr")));

namespace WebAppContext.Web
{
    public class ApplicationContext : DbContext
    {
        public ApplicationContext()
        {         
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(@"...");
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.ApplyConfiguration(new Dic1Configuration());
            modelBuilder.ApplyConfiguration(new Dic2Configuration());
            modelBuilder.ApplyConfiguration(new Dic3Configuration());
        }
    }
}

namespace WebAppContext.Dictionary
{
    public class Dic1Configuration : IEntityTypeConfiguration<Dic1>
    {
        public void Configure(EntityTypeBuilder<Dic1> builder)
        {            
        }
    }
}

namespace WebAppContext.Book.Areas.Book.Controllers
{
    [Area("book")]
    public class HomeController : Controller
    {
        public HomeController(ApplicationContext context)
        {
            // ApplicationContext ??
        }
        
        public IActionResult Index()
        {
            return View();
        }
    }
}