How were servants to the Kaiser of Imperial Germany treated and where may I find more information on them?

Another source about not just the Kaiser's but also his sons' servants (in passing, not as topic per se)

Robert Graf von Zedlitz-Trützschler: Zwöf Jahre am deutschen Kaiserhof. Aufzeichnungen des Grafen Robert Zedlitz, Trützschler ehemaligen Hofmarschalls Wilhelms II Deutsche Verlags-Anstalt Stuttgart Berlin und Leipzig 1923

Wow, This is a question I can answer. My great great great grandfather was a manservant to Kaiser Whilhelm II, and near the end of the war, he rewarded him and others with plots of land to settle in Illinois.

I personally never met him as he died nearly 70 years before I was born, but the stories my grandfather shared with me have been amazing to hear.

I was lent a copy of George, Nicholas and Wilhelm: Three Royal Cousins and the Road to World War I, and rather enjoyed the read. While I don't remember a lot of material specifically about servants, it was flat out about the behavior and mindset of those three men.

Suffice to say the picture that it painted of Wilhelm was definitely one of someone for whom that behavior wouldn't be at all out of character. Rather than just list out his purported bad qualities, I'll just say the character of King Joffre in Game of Thrones might well have been partially modeled on Carter's portrayal of Wilhelm (the timing is wrong though. A Clash of Kings predates this book by a decade). That's certainly what I thought of immediately when I watched it on HBO.

Wilhelm II was prone to fits of rage and that he would beat, or even stab, his own servants.

This is essentially true, but misleading nonetheless.

He was prone to express anger in rage, but more characteristically his default strategy for any type of conflict was yelling before and during a conflict, but then "disengagement", just like after the war when he fled to the Netherlands.

The misleading part in the summary is that it might be read as his fits of rage resulting in beating or stabbing his personnel.

That he probably wasn't the best master for his servants is also true, but especially in his younger years he didn't really beat the daylights out of servants for misdeeds or unwanted behaviour. A simple slap seemed to be quite undignified but he interpreted his position as giving him the right to do so.

He was quite fond of what he viewed as pranks on his servants. "Stabbing" evokes the picture of broad sword or at least stiletto. But what is meant by that is really the usage of needles, on chairs on pillows for example, so as to draw some joy form someone yelling ouch.

While one would still call that sadistic, it is quite different from torturing or killing a servant.

And remember his age when anecdotes like this come around:

“Willy is a dear, interesting charming boy,” Vicky wrote when he was seven, “clever, amusing, engaging, it is impossible not to spoil him a little. He is growing so handsome and his large eyes have now and then a pensive, dreamy expression and then again they sparkle with fun and delight.”

He could also be aggressive and difficult. He hit his nurses; after a trip to England in 1864, his grandmother complained that he was thumping his aunt Beatrice—who was only two years older and afraid of him. “We have a gt. deal of trouble to keep him in order—he is so jealous of the Baby,” Vicky wrote after the birth of his sister Charlotte.

Aged seven or so, on the beach at the Isle of Wight, he threw a furious tantrum and tried to kick an eminent gentleman and throw his walking stick into the sea. (The eminent gentleman, a former secretary of Prince Albert’s, tripped him up and spanked him.)

On another occasion, at his uncle Edward’s wedding in England in 1863, aged four, he got bored, scratched the legs of his uncles Leopold and Arthur to get their attention, threw his sporran into the choir, and when scolded, bit one of his uncles in the leg.* W. P. Frith, celebrated painter of crowd scenes such as Derby Day, who had been commissioned to paint the occasion, muttered, “Of all the little Turks he is the worst.” To modern eyes, this seems like fairly typical obstreperous, spoilt toddler behaviour, but at the time it struck his mother and the British relatives as more than that—though this may have been just as much to do with their impossible expectations of how a young monarch-to-be should behave.
_{Miranda Carter: "George, Nicholas and Wilhelm: Three Royal Cousins and the Road to World War I", Knopf: New York, 2009.}

Not that much of this kind of behaviour was seen later in adulthood. It is also noteworthy that not only because of high expectations of him he was really tortured, by 'medical' professionals, with good intentions, but negative outcome in all respects.

His prankish physical attitudes in later life were quite a bit more settled, but his unstable behaviour was not:

If Wilhelm lost at tennis, he would fling his racket, and he spoke nastily to or about people (or countries, notably England) who had annoyed him, sometimes so recklessly that his courtiers nearly came to blows with him. Servants in the palace on occasion refused to wait on the Kaiser because of his rages, and on a higher plane, his fury could reduce military stalwarts to tears. (as young adult)

When the Kaiser was not reading, he talked on and on, often in an alarmingly inane manner, and even though the cruise was supposed to promote relaxation, the Kaiser was often nervous or petulant, so much so that Dr. Leuthold became concerned. Wilhelm might on occasion denounce his subjects as the "dirty, obstinate, stupid, poor, ill-bred Germans, who should let the English or the Americans be their example." (in office, while yachting before the war)

Everyone found serving the loquacious Kaiser very taxing, and the adjutants and physicians, who received no pay other than their railroad fares to and from Holland, for the most part rotated duty every three months and after a few years usually opted to return permanently to Germany. An indication of how difficult it was to remain constantly exposed to the Kaiser is pathetically apparent in the departure of "Vater" Schulze, who after forty-three years decided in October 1922 that he could take no more and fled Doorn in tears without taking leave of his master of so many decades.

Of all the Kaiser's staff, the only person who was on permanent duty at Doorn was Sigurd von Ilsemann, a young officer who in August 1918 had become one of Wilhelm's adjutants and had crossed over the border to Holland in the Kaiser's automobile three months later. The handsome and polished Ilsemann in October 1920 married Count Bentinck's daughter, who, like her husband, kept a very informative diary. Ilsemann recognized Wilhelm's unfortunate qualities and often chafed at the suffocating regime in Doorn, but like the other members of the retinue, he was not inclined to register much opposition. The crown prince was correct in reproaching his father's adjutants in Doorn for their sycophancy and for thereby perpetuating Wilhelm's "world of illusion." Certainly no corrective could be expected from the Kaiserin and her entourage, the chief fixture of which was Countess Mathilde von Keller, who had served Augusta Victoria since Dona's marriage in 1881.
_{Lamar Cecil: "Wilhelm II. Vol. 2. Emperor And Exile, 1900-1941", University of North Carolina Press: Chapel Hill, London, 1996.}

Some of his servants kept diaries. One such would be:

Paul Schönberger & Stefan Schimmel: "Kaisertage. (Unveröffentlichte Aufzeichnungen aus den Jahren 1913 bis 1918 der Kammerdiener und Adjudanten Wilhelms II", Südverlag: Konstanz, 2018.

Mathilde Gräfin von Keller: "Vierzig Jahre im Dienst der Kaiserin: Ein Kulturbild aus den Jahren 1881–1921", Koehler & Amelang: Leipzig, 1935.