When using DataContract it is advisible to use Public properties instead of protected. That is my other change ... Following is the serialized datacontract with sample data.

<ArrayOfBook xmlns="http://schemas.datacontract.org/2004/07/TestSampApp" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<Book>
<Author>author1</Author>
<IsBn>1</IsBn>
<IsType>istype1</IsType>
<Price>10</Price>
<Titel>booktitle1</Titel>
</Book>
<Book>
<Author>author2</Author>
<IsBn>2</IsBn>
<IsType>istype2</IsType>
<Price>20</Price>
<Titel>booktitle2</Titel>
</Book>
</ArrayOfBook>

As per the subject, problem using DataContractSerializer, I have updated your contract with public properties. And the DataContract is getting serialized.

I have done some modification for your code -

[KnownType(typeof (FaktaBok))]
[KnownType(typeof (BarnBok))]
[KnownType(typeof (SportBok))]
[DataContractAttribute()]
public class Book
{
[DataMember]
public int IsBn = 0;

    \[DataMember\]
    public string Titel = null;

    \[DataMember\]
    public string Author;

    \[DataMember\]
    public int Price;

    \[DataMember\]
    public String IsType = null;

}

public void SerializerTestMethod()
    {
        List<Book> books = new List<Book>
                               {
                                   new Book()
                                       {
                                           Author = "author1",
                                           IsBn = 1,
                                           IsType = "istype1",
                                           Price = 10,
                                           Titel = "booktitle1"
                                       },
                                   new Book()
                                       {
                                           Author = "author2",
                                           IsBn = 2,
                                           IsType = "istype2",
                                           Price = 20,
                                           Titel = "booktitle2"
                                       }
                               };
        string writeTo = @"FilePath";

        DataContractSerializer xs2 = new DataContractSerializer(typeof (List<Book>));

        using (Stream s2 = File.Create(writeTo))
        {
            xs2.WriteObject(s2, books);
        }
        List<Book> outBook = null;
        if (File.Exists(writeTo))
        {
            using (Stream s2 = File.OpenRead(writeTo))
            {
                outBook = (List<Book>) xs2.ReadObject(s2);
            }
        }
        
    }

} }