Serious historians generally agree that Stalin's famine (not just in Ukraine) killed millions; the number you refer to ("40 million") is very high (substantially in excess of the population of the Ukraine)-- it would be high even for the famine across the entire Soviet Union; 2 to 6 million seems a likely number in the Ukraine (and note the range, because of our limited sources, uncertainty is still in the millions) -- though hunger and its effects would have been felt by many more. As you're probably aware, the subject is politically controversial, with a lot of heat, note that Ukraine defined the holodomor as a "deliberate act of genocide" in 2006, and made denial of this contention a crime.

The Ukrainian view has been that the holodomor is a "Ukrainian holocaust" -- same genocidal intent, same imputation of bad motives to those who raise questions as with the Nazi holocaust. I don't think it's wrong to say that this is too strong a position. Whatever Stalin was, I think it's fair to say that a Nazi style project of ethnic genocide was not some long term plan of his; to read his own writing from his earliest days, he's clear enough that he thinks "nationalities" will disappear in the new Marxist state, speaking of Jews, he writes:

In brief, the Jewish nation is coming to an end, and hence there is nobody to demand national autonomy for. The Jews are being assimilated.

Stalin: Marxism and the National Question [1913]

. . . and while you'll note that there are some objective differences between the Jews -- without a "closed homeland-- and the Ukrainians, it's fair to say that Stalin is clear from his earliest days about his hostility to ethnic nationalisms. But his program seems to have been, from the earliest times, a political one of assimilation, not an intentionally murderous one of genocide; he was more than ready to kill, but there's no sign in his writing that extinguishing a people was his goal.

But you didn't ask to hear what I think about this, here's a review of current historical opinion:

There are two very recent works that examine the issue with reference to the most recent archival research:

Anne Applebaum Red Famine: Stalin's War on the Ukraine

Applebaum is the most prominent and well sourced exponent of the "Stalin really meant to starve Ukraine as a political weapon" argument. She's mostly got secondary sourcing of archival materials, but she assembles them carefully; note her excellent historiographical chapters, which detail Soviet efforts to minimize and Ukrainian nationalist efforts to spin in their preferred direction. Some critics of her argument note that she's a consistent critic of the Soviet historical record, and try to discount her work as "biased", but even folks who've had questions about her work on the Gulag have had complimentary things to say about this work; for example the noted Australian historian Sheila Fitzpatrick reviewed Red Famine well in the Guardian

Against Applebaum's "Stalin meant it" argument, we have Stephen Kotkin of Princeton, who's done a massive three volume biography of Stalin from a really deep dive into archival sources. It's in the second volume of the biography

Stalin: WAITING FOR HITLER, 1929-1941

that you find his examination of the demography and politics of the famine.

Kotkin's view is that Stalin's famine extended across the nation, and was the result of incompetence and indifference in agriculture. As the Soviets pushed farmers off their land into collective farms, they destroyed traditional methods of agriculture, and produced a nationwide disaster. Kotkin notes that the largest population losses, as a percentage of population, weren't in Ukraine but in Kazakhstan. He _doesn't_ say "there was no holodomor"; just that famine was sadly seen across the Soviet Union at the time.

As a controversy: If you Google this topic, you'll find an internet battleground, Ukrainian nationalist, Russian apologists papering all sorts of discussion forums. As Ukraine tries to maintain its independence against a hostile Russia today, you can see why this is a topic that attracts a lot of attention that is more political than scholarly.

One question that is asked is "why can't we document it as clearly as the holocaust" -- and the answer there is that as killers, the Nazis were remarkably efficient bureaucrats. There's a massive paper trail of trainloads of deportees, arrivals at camps and so on. Stalin's Russia was nothing like that; his archives weren't captured by invading armies, nor were his henchmen prosecuted at war crimes trials, and in many cases the Soviets took care not to generate compromising data in the first place-- as Applebaum notes, being a statistician or demographer was itself a dangerous profession.

Conclusion:

What can a fair minded person conclude? That millions starved and that Stalin either meant for them to do so, didn't care, or tacitly approved when he found out that they were starving. Is any apologia for Stalin possible? His defenders will note that there were emergency grain purchases; but a skeptic will note "not enough to keep millions from starving"

Sources:

Population Loss in Ukraine in the 1930s and 1940s - careful demography

Investigation of the Ukrainian Famine, 1932-1933 : report to Congress / Commission on the Ukraine Famine. - older report to the US Congress, a good guide to the sources, such as were known at the time 1988.

The Foreign Office and the famine: British documents on Ukraine and the great famine of 1932-1933

- as the name suggests, these are contemporary British diplomatic reports, giving a sense of what an informed observer could see at the time

Stalinism Triumphant: Famine, Terror, And Hitler's Shadow, 1929-1941

- this is Stephen Kotkin interviewed at the Hoover Institution in December 2017; serves as a good introduction to his books, which obviously have more detail and will consume more of your time.

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