Below works for me, in Javascript Native code for MongoShell.
You can translate to C#
db.books.find({},{"Tags":{$slice:1},_id:1})
EDIT
What about this?
var query = Query.EQ(<My condition here>);
var slice = Fields.Slice("Tags", 1);
var fields = Fields.Include("_id","Tags");
return Books.FindAs<MyBookClass>(query).SetFields(fields).SetFields(slice).ToList();