Can someone remind me of the name of the author, the name of the hero and the name of the series if any?
This sounds like the Dumarest of Terra series by the English writer E. C. Tubb; the hero is Earl Dumarest. There is a Wikipedia page for the series. The quotations below are from the FAQ at Dumarest.com, except for an extract from a review at MPorcius Fiction Log.
Adventure Sci-Fi books . . . probably published late 50s?, early 60s? by Bantam? or Del Rey? I think at least 30, possibly more than 40 books in the series.
There seem to be 33 books, most of them published by Ace Books (#1-8, the first seven as Ace Doubles) or DAW Books (#9-31) in the United States. First publication dates range from 1967 for #1, The Winds of Gath, to 2008 for #33, Child of Earth.
. . . a secret potion that allows a person to change bodies with someone else and thus control them. . . . I remember the following detail about the potion: there are two nearly identical molecules that have to be assembled in a certain sequence. Two of the components had to be switched around and that determined who would be dominant and who would be controlled during the mind switch / body swap.
From Kalin he unknowingly obtained the knowledge of the “affinity twin”, a molecular
sequence of genetic units that, when properly assembled and injected into the brain cortex
of two living beings (Dumarest has even used the affinity twin on a large space alien),
allows the “master” to control the body of the “servant” and experience the world through
all of its senses. When the “servant” dies, the intelligence is returned to the “master”.
A powerful organization does not want the secret to be known, because they use the potion to control politicians on many planets.
That organization is the Cyclan:
The affinity twin formula was stolen from the laboratories of the Cyclan by Kalin’s first husband, and the Cyclan are bent on recovering the correct sequence of the formula. The Cyclan already possess knowledge of the 15 molecular units, but placing them in the correct sequence and testing them could take the Cyclan over 4,000 years, so having learned that Dumarest has the information on the correct sequence, the Cyclan have been hunting him in order to recover such information.
The organization's goons are people who had surgeries to remove their emotions, and function like organic computers. They can communicate over interstellar distances and follow and murder anyone who learns the secret.
The Cyclan is an organization composed by many logic-driven cybers who are developed
when young men undergo an operation at puberty which removes their capacity to feel
emotions and are engrafted with the Homochon elements (which are apparently obtained
from blind creatures which live in some underground caverns, possibly on Earth). Mentally
reviewing a particular complex mathematical equation, a cyber can place himself in a
trance-like state which activates the dormant Homochon elements and thus permits him to
have instantaneous communication across light year distances with Cyclan headquarters.
One book in the series starts like this: hero is on very hot planet, diving for valuable marine life, catches a deadly fungus disease and is rescued last moment in exchange for his catch...
That sounds like #5, The Jester at Scar, reviewed by MPorcius Fiction Log:
Scar is a planet with an ecology and an economy based on fungi, and Tubb relates to us all the details on how people have to adapt to live on Scar, how people make a living on Scar, the means of harvesting and the industrial uses of the many kinds of fungi, and so forth.
One of the themes that returns often is that the hero is an excellent knife fighter.
He is not particularly muscular but is blessed with extraordinary reflexes (even
when young) and a special sensitivity to circumstances and opportunities which is often
described as “luck”. . . . Dumarest has been employed in a number of odd jobs but, when down on his luck, he sometimes resorts to participating in publicly staged fights featuring knives or unarmed combat with another opponent, and virtually always Dumarest wins the match and promised reward (except in Angado).
Of course he's a womanizer
The grown Dumarest is especially attractive to women, and indeed many of the early books
in the series are titled after the leading female character (e.g., Derai, Kalin, Veruchia) with whom he develops a relationship.
In the second (or close to 2nd) book in the series, he's involved with a woman who rules a planet.
The second book is Derai. Derai is one of Dumarest's great loves, but I can't tell from the MPorcius Fiction Log review if she is a planetary ruler or not. In one of the books Dumarest is involved with a woman who not only rules, she is a planet:
Dumarest has even been loved by a sentient world, which created a
female human simulacrum to interact with Dumarest.
To travel to another world, he usually takes the cheapest dead-sleep passage, and there's always a 10-25% risk you won't wake up from that.
Rich travellers often travel “high”, which means they are injected with a drug (“slow time”)
that subjectively slows down their perception of time and biological processes so that a
trip of 20 light years can take place in about two weeks or so. Poor travellers have to opt
for “low passage”, which means they are doped up and ride in casket-type containers which
slow their metabolism (they ride 90% dead), which often proves fatal to such travellers
(15% mortality rate). Most spacers travel in “Medium”, which means that they are neither
slowed down nor doped up and stored away.