WORTH NOTING ON TV

SUNDAY

Ghost Writer (PBS, 6-7 p.m.): When "Sesame Street" introduced a generation of tots to the alphabet and reading more than 20 years ago, it radically changed viewers' notions of what "educational TV" could be. Now that series' creator, Children's Television Workshop, is launching its most ambitious project since then. It's a $21-million, 42-part mystery-adventure designed to help kids leap "the fourth-grade wall," the obstacle that researchers say many children face between learning how to read and actuall y using that skill to pursue other kinds of knowledge. The drama format sets a cast of resourceful youngsters to solving mysteries, with the aid of clues from a ghost who communicates only through writing. The multiethnic cast is hip, the Brooklyn setting realistic, the production style high-tech and high-energy (clue: executive producer is Lyn Nealon, one of MTV's originators). MONDAY

Realms of the Russian Bear (PBS, 8-10 p.m.): Ever seen a hungry polar bear leap onto the back of a walrus as the walrus plunges into the sea? It's one of the sequences from this six-parter that begins "Nature's" 11th season. In light of the challenges he faced, producer John Sparks tells me "It would have been easier to do a series on the surface of the moon." But the result is a impressive study of the ex-Soviet Union's vast, wondrous, and often overlooked natural history, from the Caspian Sea to the P acific Ocean. The series is co-hosted by George Page and his Russian counterpart, author and natural history professor Nikolai Drozdov. (Part 2 and 3 air Tuesday and Wednesday; each program offers two episodes.) Please check local listings for all PBS programs.