Martin's Hats, by Joan M. Blos. Illustrated by Marc Simont. New York: William Morrow & Co. Pages unnumbered. $9. While all the action in this exquisitely illustrated book takes place within a small boy's imaginings as he sits in his bedroom, readers will find themselves eagerly joining in Martin's life-affirming spirit.
From his first adventure - when he dons a ''real'' explorer's cap - to his last, complete with snuggly wool nightcap, Martin wears as many as 10 different hats. Whether it's a wedding picnic, playing the role of a T-shirted conductor on an old-style locomotive, or welding a girder high among the skyscrapers, Martin's hats aren't so much playthings as invitations to explore a number of lives. The book is really a display of boyish imaginings, all hanging together somehow with remarkable cohesiveness - the hats being the ''glue'' of the myriad experiences.
It's not only himself whom Martin pictures wearing different hats. Most of the people he imagines throughout the book have their own favorite hats, too - all of which give, very cleverly, an inkling into each wearer's distinctive personality.
Martin's dreams are what a lot of children fantasize about: heroes saving the day! experts on the job! It's healthy, constructive musing which, in the case of this book, has the added virtue of illustrating a wide-ranging love of life, with the effect of complementing, not risking, one's own individuality.