Ramona is back; Ramona Quimby, Age 8, by Beverly Cleary, illustrated by Alan Tiegreen. New York: Morrow Junior Books. $7.95. 190 pages.

Irrepressible Ramona Quimby is at it again.

As a new third grader, having ''reached the age of demanding accuracy from everyone, even herself,'' Ramona spends much of her time asking questions. She also has to learn a lot of third-grade ''do's'' and ''don'ts,'' such as how to poke holes in oranges to suck out the juice, since third graders do not peel their oranges. And when it comes to the latest fad of cracking open hard boiled eggs on heads, she runs into trouble enough for the entire class.

Ramona's believable adventures always have an instructive purpose in Beverly Cleary's caring hands, however. She deals with such young concerns as getting ill in class and worrying about parents losing jobs. Although the Quimbys are a ''nice sticking-together family'' in Ramona's eyes, they have their share of contemporary challenges: her mother works full-time and runs the household on a shoestring budget (which includes ''yucky'' beef tongue for dinner) so that her father can return to college for an art degree.

One of the many strengths of this book is author Cleary's quiet encouragement of reading. Ramona can find short words in long words, such as ''earthquakes,'' and she reads well enough to be able to spot ''the dreaded words, 'Child's Plate' '' on a restaurant menu. What's more, her favorite period of school comes after lunch, when the third graders are allowed to read any books they like - even mysteries and fairy tales - by themselves during Sustained Silent Reading. No word lists, no summaries, no book reports are required. It's just a time to enjoy.