Hello Everyone,

I have an email template which I want to send by apex class.I am getting an Email notification but merge field's value is not showing.I tried many solutions but not getting the result.Please, can anyone help me?
Here is the snap of my email template


And apex code:-
 

global class sampleBatch implements Database.Batchable<sObject>{
global Database.QueryLocator start(Database.BatchableContext bc) {
    String query = 'Select id,Name from book__c;
    return Database.getQueryLocator(query);
}

global void execute(Database.BatchableContext bc, List<book__c> books){ 
    Map<Id, List<Case>> associatedCases = CaseHelperClass.getCase(new Map<Id,book__c>(books).keySet());
    Map<Id,book__c> bookMap= new Map<Id,book__c>(books);
    EmailTemplate emailTemplate = [SELECT Id,Subject,Description,HtmlValue,DeveloperName,Body FROM EmailTemplate WHERE DeveloperName =: 'sameple_template'];
    if(associatedCases <> null){
        for(Id bookId : associatedCases.keySet()){
            String headerAndAssociatedCases = 'CaseNumber,Type,Status,Subject,Description,CreatedDate\n';
            for(Case c : associatedCases.get(bookId)){
                headerAndAssociatedCases += c.CaseNumber+','+c.Type+','+c.Status+','+c.Subject+','+c.Description+','+c.CreatedDate+'\n';
            }
            Messaging.EmailFileAttachment csvAttachment = new Messaging.EmailFileAttachment();
            blob csvBlob = Blob.valueOf(headerAndAssociatedCases);
            csvAttachment.setFileName('Case.csv');
            csvAttachment.setBody(csvBlob);
            Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
            list<string> toAddresses = new list<string>{name@test.com};
            email.setSubject(emailTemplate.Subject);
            email.setTemplateId(emailTemplate.Id);
            email.setPlainTextBody(emailTemplate.Body);
            email.setToAddresses(toAddresses);
            email.setFileAttachments(new Messaging.EmailFileAttachment[]{csvAttachment});
            Messaging.SendEmailResult [] result = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
       }
    }
}

global void finish(Database.BatchableContext bc){
    system.debug('Apex Job Done');
}}

Any help will be highly appreciable. Thank you!