import { AxiosInstance, AxiosError } from "axios";
/**
 * Available subscription plans on ISBNdb.
 * - BASIC: 1 req/sec
 * - PREMIUM: 3 req/sec — uses https://api.premium.isbndb.com
 * - PRO: 5 req/sec — uses https://api.pro.isbndb.com
 */
export type IsbndbPlan = "BASIC" | "PREMIUM" | "PRO";
/**
 * Creates an Axios instance preconfigured for the ISBNdb API.
 *
 * @param apiKey Your personal API key from isbndb.com
 * @param plan The plan level to determine the correct base URL. Defaults to 'BASIC'
 * @returns AxiosInstance preconfigured with baseURL and headers
 */
export declare function createIsbndbClient(apiKey: string, plan?: IsbndbPlan): AxiosInstance;
/**
 * A standardized error structure for all failed API calls.
 */
export interface IsbndbError {
    status: number;
    statusText?: string;
    message: string;
    url?: string;
}
/**
 * Formats an AxiosError into a more readable IsbndbError.
 *
 * @param error AxiosError from the request
 * @returns IsbndbError object
 */
export declare function formatIsbndbError(error: AxiosError): IsbndbError;