點 is a bit or a touch, and 心 is heart (the character looks like a heart), so 點心 can be translated as a bit of heart or a touch of heart or touching the heart (Grace Young in The Breath of a Wok translates it as a dot on the heart). Anyway, the origin of the term is debated.
點 is also the first character in why (點解 dím gáai) and o'clock (點鐘) dím jūng. 心 is also the second character in be careful! (小心 síu sām), happy (開心 hōi sām) and the vegetable choy sum (菜心 choi sām).
Dim sum are shared small dishes or steamer baskets of dumplings and other typically bite-sized foods.
History
Dim sum dishes originated from all over China and some even have Western influences, but dim sum as a Hong Kong and Cantonese cuisine (The English spelling is close to the Cantonese pronunciation dím sām) originated in the yum cha (drink tea) tradition of travelers stopping at teahouses for tea and snacks along the Silk Road (so dim sum was an early version of truck stop food).
Dim sum is known for being served in on carts in large restaurants, but these days it can be found in a variety of settings ranging from banquet-sized restaurants to cozier Hong Kong-style cafes (cha chaan teng) and ordered off a menu.
Books
The Dim Sum Field Guide by Carolyn Phillips is an illustrated overview of dim sum, Dim Sum: The Art of Chinese Tea Lunch by Ellen Leong Blonder is an illustrated collection of recipes and some background info. Dim Sum by Janice Wong and Ma Jian Jun presents recipes for traditional and new dim sum with a focus on the flour used in the wrappings. The Nom Wah Cookbook by Wilson Tang features recipes from the Nom Wah Tea Parlor, a New York Chinatown institution, and profiles the immigrants and their descendants who've kept multi-generational businesses alive in Chinatown. Grace Young's The Wisdom of the Chinese Kitchen includes some dim sum recipes.
Books for Kids
Dim Sum for Everyone! by Grace Lin is an illustrated dim sum family outing. Dim Sum Time by Sin Siu Tang and Andrew Sun teaches Cantonese dim sum phrases.