01_BookTrigger.trigger

trigger BookTrigger on Book__c (after insert, after update) {
    
    BookTriggerHandler handler = new BookTriggerHandler();
    
    if (Trigger.isBefore) {
        // before trigger logic
    } else if (Trigger.isAfter) {
        if (Trigger.isInsert) {
            // GoogleBooksAPIで書籍の情報を取得
            handler.setGoogleBooksInfo(Trigger.new);
        } else if (Trigger.isUpdate) {
            // GoogleBooksAPIで書籍の情報を取得
            handler.setGoogleBooksInfo(Trigger.new, Trigger.newMap, Trigger.oldMap);
        } else if (Trigger.isDelete) {
            // delete trigger
        } else if (Trigger.isUnDelete) {
            // undelete trigger
        }
    }
}

02_BookTriggerHandler.cls

public with sharing class BookTriggerHandler {
    
    private BookTriggerAction action = new BookTriggerAction();
    
    /**
     * コンストラクタ
     */
    public BookTriggerHandler() {
        
    }
    
    /**
     * GoogleBooksAPIで書籍の情報を取得
     */
    public void setGoogleBooksInfo(List<Book__c> books) {
        // 対象のBook ID
        Set<Id> bookIds = this.action.getBookIds(books);
        if (bookIds.isEmpty()) return;
        
        // 非同期でGoogle Books APIの処理実行
        BookTriggerFuture.doCalloutGoogleBooksApi(bookIds);
    }
    
    /**
     * GoogleBooksAPIで書籍の情報を取得
     */
    public void setGoogleBooksInfo(List<Book__c> books, Map<Id, Book__c> newBooksMap, Map<Id, Book__c> oldBooksMap) {
        // 対象のBook ID
        Set<Id> bookIds = this.action.getBookIds(books, newBooksMap, oldBooksMap);
        if (bookIds.isEmpty()) return;
        
        // 非同期でGoogle Books APIの処理実行
        BookTriggerFuture.doCalloutGoogleBooksApi(bookIds);
    }
}

03_BookTriggerAction.cls

public with sharing class BookTriggerAction {
    
    /**
     * コンストラクタ
     */
    public BookTriggerAction() {
        
    }
    
    /**
     * Book ID取得
     */
    public Set<Id> getBookIds(List<Book__c> books) {
        Set<Id> bookIds = new Set<Id>();
        for (Book__c b : books) {
            bookIds.add(b.Id);
        }
        
        return bookIds;
    }
    
    /**
     * Book ID取得
     */
    public Set<Id> getBookIds(List<Book__c> books, Map<Id, Book__c> newBookMap, Map<Id, Book__c> oldBookMap) {
        Set<Id> bookIds = new Set<Id>();
        for (Book__c b : books) {
            if (newBookMap.get(b.Id).Name != oldBookMap.get(b.Id).Name) {
                bookIds.add(b.Id);
            }
        }
        
        return bookIds;
    }
}

04_BookTriggerDao.cls

public with sharing class BookTriggerDao {
    
    /**
     * IDを条件にBook情報を取得
     */
    public List<Book__c> getBooks(Set<Id> bookIds) {
        return [
                SELECT
                     Id
                    ,Name
                    ,Price__c
                    ,Web_Results__c
                FROM
                    Book__c
                WHERE ID IN: bookIds
        ];
    }
}

05_BookTriggerFuture.cls

public with sharing class BookTriggerFuture {
    
    /**
     * 非同期処理でGoogleBooksApiから書籍情報を取得
     */
    @future (callout=true)
    public static void doCalloutGoogleBooksApi(Set<Id> bookIds) {
        
        BookTriggerAction action = new BookTriggerAction();
        BookTriggerDao dao = new BookTriggerDao();

        // IDを条件にBook情報取得
        List<Book__c> books = dao.getBooks(bookIds);
        Map <String, Book__c> bookMap = new Map<String, Book__c>();
        // ※NameをKeyにするとユニークにならないので要検討...
        for (Book__c b : Books) {
            bookMap.put(b.Name, b);
        }
        
        //
        // Google Books APIを使った処理...(省略)
        // 
        
        // APIで取得したGoogleBooksの情報を条件にBook情報を更新
        List<Book__c> updateBooks = new List<Book__c>();
        for (String key : bookMap.keySet()) {
            Book__c wkBook = bookMap.get(key);
            wkBook.Price__c = 777777;       // Priceに値をセット（サンプル）
            updateBooks.add(wkBook);
        } 
        
        update updateBooks;
    }
}