"Merge two payload into one in transform component "

I am trying to add two json payload into one but getting error. I am using scatter gather where I am getting multiple payload and combining the payload into transform message. Error is: Exception while executing: payload[0] map (bookListing) -> using (id = bookListing.bookId) { Type mismatch for 'map' operator found :string, :function required :array, :function.

code:

      <http:listener-config name="HTTP_Listener_Configuration" host="0.0.0.0" port="8081" doc:name="HTTP Listener Configuration"/>
<file:connector name="file-connector-config" autoDelete="false" streaming="true" validateConnections="true" doc:name="File" />
<flow name="muleRequester">
    <http:listener config-ref="HTTP_Listener_Configuration" path="/requester" doc:name="HTTP"/>
    <scatter-gather doc:name="Scatter-Gather">
        <processor-chain>
            <mulerequester:request resource="file://src/main/resources/input1" returnClass="java.lang.String" doc:name="Retrieve File1"/>
            <logger message="#[payload]" level="INFO" doc:name="Logger"/>
        </processor-chain>
        <processor-chain>
            <mulerequester:request resource="file://src/main/resources/input2" returnClass="java.lang.String" doc:name="Retrieve File2"/>
            <logger message="#[payload]" level="INFO" doc:name="Logger"/>
        </processor-chain>
    </scatter-gather>
    <dw:transform-message doc:name="Transform Message">
        <dw:set-payload><![CDATA[%dw 1.0
         %output application/json
         ---
         payload[0]  map (bookListing) -> using (id = bookListing.bookId) {
bookId: id,
title:  bookListing.title,
price:  bookListing.price,
(payload[1] filter ($.*bookId contains id)  map (bookAuthor) -> {
    author:bookAuthor.author
        })
    }]]></dw:set-payload>
     </dw:transform-message>
     <file:outbound-endpoint responseTimeout="10000" doc:name="File" outputPattern="#[function:systime].json" path="src/main/resources/output"/>
    <logger message="#[payload]" level="INFO" doc:name="Logger"/>
</flow>

payload 1 - [ { "bookId":"101", "title":"world history", "price":"19.99" }, { "bookId":"202", "title":"the great outdoors", "price":"15.99" } ]

payload 2- [ { "bookId":"101", "author":"john doe" }, { "bookId":"202", "author":"jane doe" } ]

Expected output-

[ { "bookId": "101", "title": "world history", "year": "2017", "isbn": "11111", "price": "19.99", "author": "john doe" }, { "bookId": "202", "title": "the great outdoors", "year": "2016", "isbn": "22222", "price": "15.99", "author": "jane doe" } ]

The values returned from the files are just string so need to read them into something that can be used by dataweave first. Use the read function like so:

read(payload[0], 'application/json')

and

read(payload[1], 'application/json')

Here is a full example. (I have stubbed out the sample json in the scatter-gather):

    <scatter-gather doc:name="Scatter-Gather">
       <set-payload value="[ { &quot;bookId&quot;:&quot;101&quot;, &quot;title&quot;:&quot;world history&quot;, &quot;price&quot;:&quot;19.99&quot; }, { &quot;bookId&quot;:&quot;202&quot;, &quot;title&quot;:&quot;the great outdoors&quot;, &quot;price&quot;:&quot;15.99&quot; } ]" doc:name="payload1"/>
       <set-payload value="[ { &quot;bookId&quot;:&quot;101&quot;, &quot;author&quot;:&quot;john doe&quot; }, { &quot;bookId&quot;:&quot;202&quot;, &quot;author&quot;:&quot;jane doe&quot; } ]" doc:name="payload2"/>
    </scatter-gather>

    <dw:transform-message doc:name="Transform Message">
       <dw:set-payload><![CDATA[%dw 1.0
%output application/json
---
read(payload[0], 'application/json')  map (bookListing) -> using (id = bookListing.bookId) {
bookId: id,
title:  bookListing.title,
price:  bookListing.price,
(read(payload[1], 'application/json') filter ($.*bookId contains id)  map (bookAuthor) -> {
    author:bookAuthor.author
})
}]]></dw:set-payload>
   </dw:transform-message>

Or you could read the whole thing into Json before hand