One of many easy mystery series that readers tell apart chiefly by their identifying gimmicks, Singer's is the one with the twin brother sleuths. But if Singer's writing is typically mechanical, Judy Glasser's oblique, atmospheric pictures provide some offbeat charm and style. Of this season's two entries, A Clue in Code poses a standard kids'-mystery problem--who stole the class trip money--and duly scatters suspicion among several of Sam and Dave's classmates. In the course of the twins' investigation, a coded message hits Sam on the head (it is written on a paper airplane) and a valuable comic-book collection gets mixed up with the missing money. For the genre, the solution is sufficiently well prepared and unexpected. The second story is farther fetched. The Case of the Cackling Car takes the twins to Papagayo, Texas; lays on some poster-paint local color with fiesta floats and a string of Spanish-named characters; stirs things up with a kidnapping; and has Sam himself tied up in a car trunk with the kidnapped child before Dave and the twins' aunt, following in her car, catch up with the culprit--a parrot smuggler who snatched the kids when they discovered the cackling contraband in his car. This one is strictly for puzzle-story addicts who don't ask for credible motivation or for characters of even two dimensions.