The first group are the Mel Martin series. They are basically mystery stories surrounding a group of high school baseball players, Mel Martin being the main focus. I have all six of these with the dustjackets published by Books inc. It is a Stratemeyer Syndicate series of which the first two titles were originally published by Cupples & Leon in 1947. Garden City reprinted these two titles and published four new titles in 1952 and 1953. Books Inc. reprinted the entire set in the late 1950's. In the 1980's, Wanderer Books reprinted the first four volumes.

The Mystery at the Ballpark by John R. Cooper

The College League Mystery by John R. Cooper

First Base Jinx by John R. Cooper

The Phantom Homer by John R. Cooper

The Southpaw's Secret by John R. Cooper

The Fighting Shortstop by John R. Cooper
 
 

These two are rather simular to the Mel Martin stories above. High school ballplayers and mysteries combine. I found these without dustjackets and don't know what the DJs look like.

Over The Line by Harold M. Sherman

Strike Him Out by Harold M. Sherman

These are independent stories by John R. Tunis. Tunis's ballplayers tend to be older and so have older type problems. The first book is pretty old, (1948), and worn with no dustjacket. The red on the cover belongs, the blue doesn't. The second book might not have come with a dustjacket. It looks like a picture cover type, probably printed up for school libraries. The third book is in nice shape with the dustcover included.

Highpockets by John R. Tunis

Schoolboy Johnson by John R. Tunis

The Kid Comes Back by John R. Tunis

Pitcher Pollock by Hall of Famer, Christy Mathewson

The Clair Bee, Chip Hilton series was dedicated to all the major high School sports. I found this one that had Chip starring in baseball.

Strike Three by Clair Bee