Then you need to check into nested queries. Check documentation here 

http://world.episerver.com/documentation/Items/Developers-Guide/EPiServer-Find/12/DotNET-Client-API/Searching/nested-queries/

That's exactly what nested queries were meant to support...

Hi,

I think you should be able to do this using TermsFacetFor.

Create an extension method to get the Ids:

    public static class Extensions
    {
        public static List<int> Ids(this IEnumerable<Manufacturer> manufacturers)
        {
            return manufacturers.Select(x => x.Id).ToList();
        }
    }

then make sure there are indexed:

SearchClient.Instance..Conventions.ForInstancesOf<Book>().IncludeField(x => x.Manufacturers.Ids());

Remember to reindex!

Since the Id's are ints and not strings, you'll need to do a small "hack" in order to get Find to accept the input:

 .TermsFacetFor(x => (string)(object) x.Manufacturers.Ids())