IT'S EASY to be annoyed by the trend toward books as toys. After all, books should be able to entertain without resorting to battery-powered sound effects or pop-ups as intricate as origami.
With ''The Jolly Christmas Postman,'' however, Janet and Allan Ahlberg have come up with a thoroughly charming gimmick. The postman rides his bike across the countryside, delivering letters that readers can pull out of their envelopes and read for themselves.
The Ahlberg's original ''The Jolly Postman'' has sold over a million copies, and ''The Jolly Christmas Postman'' (Little, Brown and Company, $16.95, ages 3 and up) could well do the same. The lure of reading other people's mail is enticing.
First there's a Christmas card addressed to Four Bears Cottage, from Goldilocks. Miss R. Hood receives a card that doubles as a board game called Get out of the Woods! It's from a Mr. Wolf.
For Humpty Dumpty, there's a working jigsaw puzzle from all the king's horses and all the king's men. And the Gingerbread Boy receives a goofy little book of puzzles, jokes and stories from ''Daddy and all at Pat O'Cake Bakers.''
As with all of the Ahlbergs' work, the busy scenes and the attention to detail mean that you'll discover a new pun or visual gag on the fifth, 10th or 30th reading. A few of the jokes will go over kids' heads, but that's OK. It's the kind of book Mom and Dad won't mind reading aloud again and again. * Another author-illustrator team with a gift for sophisticated humor has a holiday book out: ''Christmas in July,'' story by Arthur Yorinks, pictures by Richard Egielski (Michael di Capua Books, HarperCollins, $14.95, all ages).
Yorinks and Egielski won the 1987 Caldecott Medal for ''Hey, Al,'' and also teamed up on ''It Happened in Pinsk,'' ''Oh, Brother'' and ''Ugh.'' Their latest tells what happens when Santa's red pants get lost at the North Pole dry cleaners and somehow get delivered to a penthouse in New York, where a jerk named Rich Rump (the spitting image of Donald Trump) decides he likes the pants and refuses to return them to their rightful owner.
Santa, wearing his complete outfit minus his pants, wanders the city looking for trousers. Of course he's arrested for loitering in his polka-dot boxer shorts and must spend six months in the slammer.
Christmas arrives, but Santa doesn't. Everyone waits for him to come, so no one goes to work and businesses go bankrupt. By the time Santa gets out of jail in July, the world is a mess and Rich Rump is out on the street, trying to sell his red pants for a cup of coffee. Along comes Santa. As soon as he is reunited with his pants, Christmas finally can arrive -- even in July.
* Ever wonder what Santa Claus was like as a young boy? Or why he started asking kids in department stores what they want for Christmas? Such are the burning questions addressed by Dan Greenburg, author of ''How To Be a Jewish Mother'' and ''Confessions of a Pregnant Father,'' in his new book, ''Young Santa'' (Viking, $13.95, ages 9 and up).
The jokes in this book are geared toward an adult audience, and that impression is reinforced by Warren Miller's illustrations (he's been a staff cartoonist with The New Yorker since 1961). Is it funny? Even if it makes you groan instead of guffaw, it's fast-paced and easy to flip through.
'A Calf for Christmas'
* One of last year's best holiday books was ''The Christmas Carp,'' and its illustrator, Marit Tornqvist, is back this year with ''A Calf for Christmas,'' a story written by Astrid Lindgren (R&S Books, Farrar, Straus and Giroux, $13.95, ages 5-8). Tornqvist's fanciful watercolors of the Swedish countryside and small town life capture the down-to-earth flavor of the story by Lindgren, whose 115-plus books include ''Pippi Longstocking.''
A poor farming family wakes up one morning to find its only cow is dead -- she has swallowed a nail. In the meantime, a rich farmer from down the road goes to town for his monthly drunken binge, buying a calf before he loses his faculties. On the ride home in his horse-drawn sleigh, he forgets about the calf and accidentally leaves it by the side of the road in front of the poor family's farm. As fate would have it, the poor little boy discovers the calf and his rich neighbor, hungover and contrite, decides to let him keep it.
'One Winter's Night'
* ''One Winter's Night,'' by Primrose Lockwood, illustrations by Elaine Mills (Macmillan, $13.95, ages 3-7) is actually about a birthday present for a little boy named Joseph. But the snowy season makes it just right for reading as Christmas approaches.
''In a barn somewhere a dog is barking. Joseph's father goes up to the farmhouse door. Why has he come? What is he seeking?'' The rhythmic verse keeps readers in suspense until Joseph's parents surprise him with his gift -- a border collie puppy.
'On the Way to Christmas