[SalesForce] Get data on commandbutton click from pageblocktable

I have this below code where i am parsing json response and displaying it in pageblocktable. I have a button on each row. When clicked it is calling a method. I want when clicked, it should pass the row data to that method.

Basically i am trying to get the data (tittle, author) when add button is clicked in that row.
Apex class –

public class BooksJson {
    public List<Items> items {get;set;} 

    public class Items {
    public VolumeInfo volumeInfo {get;set;}
  }

      public class VolumeInfo { 
            public String title {get;set;} 
            public String subtitle {get;set;} 
            public List<String> authors {get;set;} 
            public String description {get;set;}
            public List<IndustryIdentifiers> industryIdentifiers {get;set;}
            public ImageLinks imageLinks {get;set;}
        }

    public class IndustryIdentifiers {
        public String type_Z {get;set;} // in json: type
        public String identifier {get;set;}
    }

    public class ImageLinks {
        public String smallThumbnail {get;set;} 
        public String thumbnail {get;set;}
    }


}




public class Callout {

    public list<BooksJson.items> returnResultList { get;set; }
    public String searchText { get; set; }
    public String author { get; set; }
    public String isbn { get; set; }
    public String tittle { get; set; }

    public void search() {
        if(searchText != null) {
            bookSearch(searchText);

        }
    }

    public pagereference addBook() {

        return null;
    }

    Public void Search(String searchText){

            HTTPresult = response.getBody();
            BooksJson result = (BooksJson)JSON.deserialize(HTTPresult ,BooksJson.class);
            For(BooksJson.items g :result.items) {
                count++;
                returnResultList.add(g);
            }

        } else if (response.getStatusCode() == 500) {
        } else {
        }
    }
}

Vf Page –

<apex:page controller="Callout" sidebar="false">

    <apex:form id="form">
        <center>
            <div class="SearchDiv">
                <apex:inputText value="{!searchText}" >
                <apex:commandButton action="{!search}" value="search" reRender="form"/>
            </div>
        </center>
        <apex:variable value="{!0}" var="index" />
        <apex:pageBlock id="searchRes" rendered="{!returnResultList != null}">
            <apex:pageBlockTable value="{!returnResultList}" var="res">
                <apex:column headerValue="slNo">
                    <apex:outputText >
                        <apex:variable value="{!index + 1}" var="index" />
                        {!index}
                    </apex:outputText>
                </apex:column>
                <apex:column value="{!res.volumeinfo.title}" headerValue="Title" />
                <apex:column value="{!res.volumeinfo.authors}" headerValue="Author" />
                <apex:column headerValue="Action">
                    <apex:commandButton action="{!addBook}" value="Add">
                        <apex:param assignTo="{!tittle}" name="tittle" id="tittle" value="{!res.volumeinfo.title}"/>
                        <apex:param assignTo="{!author}" name="Author" id="author" value="{!res.volumeinfo.authors}"/>
                    </apex:commandButton>
                </apex:column>
            </apex:pageBlockTable>
        </apex:pageBlock>
    </apex:form>

</apex:page>