first_.xsd
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
     targetNamespace="www.first.com">
    <xs:simpleType name="bid">
        <xs:restriction base="xs:string">
            <xs:pattern value="[A]\d{6}"/>
        </xs:restriction>
    </xs:simpleType>
</xs:schema>

second_.xsd
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="www.second.com">
    <xs:simpleType name="aid">
        <xs:restriction base="xs:string">
            <xs:pattern value="[c]\d{6}"/>
        </xs:restriction>
    </xs:simpleType>
</xs:schema>

_third.xsd
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns="www.myweb.com"
    targetNamespace="www.myweb.com"
    xmlns:first="www.first.com"
    xmlns:second="www.second.com">
    
    <xs:import namespace="www.first.com" schemaLocation="first_.xsd"/>
    <xs:import namespace="www.second.com" schemaLocation="second_.xsd"/>
    
    <xs:element name="books" type="infotype"/>
    
    <xs:complexType name="infotype">
        <xs:sequence>
            <xs:element name="book" type="booktype"/>
        </xs:sequence>
    </xs:complexType>
    <xs:complexType name="booktype">
        <xs:sequence>
            <xs:element name="title" type="xs:string"/>
            <xs:element name="author" type="atype"/>
        </xs:sequence>
        <xs:attribute name="bookid" type="first:bid"/>
    </xs:complexType>
    <xs:complexType name="atype">
        <xs:sequence>
            <xs:element name="firstname" type="xs:string"/>
            <xs:element name="lastname" type="xs:string"/>
        </xs:sequence>
        <xs:attribute name="authorid" type="second:aid"/>
    </xs:complexType>
</xs:schema>