1952 is often thought of as Topps 1st baseball card set, but it was not.
Topps issued several smaller baseball card sets prior to their huge 1952 set.
Topps buzz word was "BIGGER is BETTER" for their 1952 Topps set which Topps described as: "GIANT IN BOTH SIZE and NUMBER of CARDS" (407).

Key card in the 1952 Topps set is #311 MICKEY MANTLE. Often called Mickey Mantle's Rookie card - BUT IT IS NOT. That honor goes to his 1951 Bowman.
1952 Topps "High Numbers" (#311-#407), are very, very scarce with an interesting story:
This HUGE set was released in series weeks apart. By the last (6th) series, baseball season was over and football starting. Candy shops had plenty of baseball cards from earlier series so most cancelled their orders for the last series creating the scarcity.

Adding interest is how Topps disposed of the now un-needed cards including THOUSANDS of 1952 Topps MICKEY MANTLE's. They dumped them into the Atlantic Ocean like most of New York's trash in those days.

1955 Topps was the first of 2 years of horizontal card designs. As with 1952, 1953 & 1954, cards were again a tad larger than today's standard. A nice feature, facsimile autographs, again appeared on the cards. The set came in 2 series, #1-160 and scarcer "High Numbers" #161-210.

Due to the contract battles between Topps & Bowman, Cards #175,186,203 & 209 were never issued. Topps eventually won the war, buying Bowman making 1955 the last year for Bowman cards. But the damage was done as Topps had to leave out many stars including Mickey Mantle & Stan Musial.

There were a total of (15) Hall of Famers in the 1955 set !!!
Top ROOKIEs: Roberto Clemente, Sandy Koufax & Harmon Killebrew
Top Stars: Hank Aaron, Ted Williams, Al Kaline, Ernie Banks, Warren Spahn, Jackie Robinson, Eddie Mathews, Willie Mays, Yogi Berra & Duke Snider.

"The Golden Greek" Harry Agganis, a popular 1955 Topps card, is a tragic story, Agganis, a rising young star, died after his first card was issued. A football star at Boston U., he turned down an offer from the Cleveland Browns and became the starting first baseman. Shortly after he came down with pneumonia and died of a Pulmonary Embolism.