BOOK REVIEW : Immigrants’ Letters Illuminate Anxiety : BETWEEN THE LINES: Letters Between Undocumented Mexican and Central American Immigrants and Their Families and Friends. <i> Translated</i> ,<i> edited by Larry Siems</i> The Ecco Press $24.95; 336 pages

In their letters, the family members and friends in Mexico or El Salvador or Guatemala write details of life at home: the birth of a goat, the breakdown of a truck. Grandma’s still sick. Thanks for the dress you sent.

The pioneers who have gone north write with an anxious edge. Did you get the check? Be sure to find someone trustworthy to cash it. Take good care of my babies, take care of my little sister, please don’t forget me. Under the expressed anxiety there’s the repeated theme: We’re here to make things better for the family. And there’s constant homesickness: The people are unkind here, and the food is terrible. And always, the fear: what if we return with nothing?

On both sides of the border, there’s the constant sadness of separation. “Mama please come because here although it’s only tortillas with beans, there’s that much, by the grace of God.”

Larry Siems, a Los Angeles writer who works with young adults at the Los Angeles Conservation Corps, has translated and arranged hundreds of letters written from 1990 to 1992 between undocumented immigrants in California and their friends and family back home. There are lively exchanges in this original and very fresh material, but the book, “Between the Lines,” is not well organized, and the reader can get lost.

One of the strong characters who holds your attention is Sylvia Martinez, a hardy young woman from Oaxaca. She describes how, after finally arriving in Los Angeles, she sought out the family friend who was supposed to help her: “Her first words were why did you come, you’re going to see how things here aren’t like you imagine them. . . .”

Martinez finds Los Angeles a sad city, where people won’t give you directions on the street or give you change to make a phone call. “Here in this city,” she writes, “they’re not a united people; here the good gambler wins.”

Another young woman, Angela Gomez, writes from Fresno to her friend Mariana Garcia Chavez in Jalisco about her depressing living arrangements.

“The apartments are in an area where there’s a mountain of drug addicts and prostitutes . . . so we’re shut up inside for almost the entire day watching television.” Momentarily discouraged, Gomez writes: “I wanted to climb out of one grave in Guadalajara and I came to fall into an even deeper one. . . .”

But she’s indomitable. She combines a tough baby-sitting and invalid-care job with volunteer work at a bilingual radio station. Her friend Mariana Garcia misses her a lot and writes, “I think it would be better if you were here in Mexico with the people who love you, and not in those inhuman . . . places that in the final analysis have given you less than what you can enjoy in our cactus-assed country.” Most kindly, Mariana tells her friend, “Don’t be afraid to return without having succeeded (in other people’s eyes) in doing something ‘productive.’ ”

Although lit up by personalities, too much of the book reads as if the letters had spilled out of a box. An introduction by Jimmy Santiago Baca is moving and literate, but it doesn’t provide the orientation we need. “Between the Lines” would be a lot more readable if Siems had supplied dates, the age of the letter writer and a little something about the writer’s situation. He has left the reader clueless at some crucial times. There’s mysterious talk of rumors and slander, for example, and of the need for forgiveness, but we don’t really know what for.

There are moments of real illumination, but the narrative is always sketchy. You lose confidence that you’ll get to the end or even the middle of a story. The masterful movie, “El Norte,” about the Guatemalan brother and sister who come to Los Angeles, shows how much easier it is to follow a single story. There are many stories here, material for many movies.

The best chapter in “Between the Lines” is the one focusing on only two families: one from Guatemala, the other from El Salvador. We see how difficult it is to manage a distant family.

Maria Amparo Ramirez, a domestic worker in Van Nuys, writes to advise her son in El Salvador to avoid the guerrillas and her 13-year-old daughter to be patient in caring for her sick and irritable grandmother. And we see how painful that distance can be. “Little brother, you don’t know how much we miss you,” writes one of the Guatemalan family members, “and now at Christmas we’re going to miss you still more.”