On a residential street three blocks from Harvard Square, this four-story gray clapboard B&B is a bargain. It has two small porches, hardwood floors, and Oriental carpets, making it homier than a hotel, and there's a real sense of conviviality among guests, mostly Europeans and visiting parents or professors. The inn maintains a giving library, which means you can take home that book you didn't quite finish. The small, superclean rooms are recently refurbished with bright new paint, flatscreen TVs, and original art. The limited off-street parking is a real coup in car-clogged Cambridge. So is the green and guilt-free solar thermal water-heating system. As a nice added touch, the Irving House will mail postcards anywhere free of charge.