Try this:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
<xs:element name="booktitles">
<xs:complexType>
<xs:sequence>
<xs:element ref="author"/>
<xs:sequence maxOccurs="unbounded">
<xs:element ref="title"/>
<xs:element ref="year"/>
</xs:sequence>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="author" type="xs:string"/>
<xs:element name="title" type="xs:string"/>
<xs:element name="year" type="xs:string"/>
</xs:schema>
Courtesy of trang, from this
<!ELEMENT booktitles
(author, (title, year)+)
>
<!ELEMENT author
(#PCDATA)
>
<!ELEMENT title
(#PCDATA)
>
<!ELEMENT year
(#PCDATA)
>