Can anyone find the letter of Tsar Alexander II to Rostovtsev dated October / December 25th, 1859?

You had the right book, but the wrong volume. The reference in your image cites:

Semenov, Osvobozhdenie krest'ian, 2:128.

The "2" in "2:128" refers to volume 2. Searching the Internet Archive for "Osvobozhdenie" in metadata and sorting the results by Date Published descending yields about 20 results. Near the end of the list, there are four items with author "Semenov, Nikolaĭ Petrovich, 1823-1904," publication year 1889, and title similar to "Osvobozhdenīe krestʹi͡anʺ vʺ tsarstvovanīe Imperatora Aleksandra II : khronika d͡iei͡atelʹnosti komisī̈ĭ po krestʹi͡anskomu d͡ielu."

Hovering over the title links, you can see that they have similar document names, and one of them has an "02" in it, so that would be a good place to start looking. In fact, the title page for this book shows ТОМЪ II or VOLUME II:

Turning to page 128 of volume 2, we see what looks like a letter signed with an "A.":

The letter starts on page 127:

PDF and TXT formats of the book are available, from which we extract this (possibly incorrect) OCRed text:

Затѣмъ, на поляхъ письма Я. И. Ростовцова Государь Императоръ написалъ ему собственноручно:

«Гатчина, 25 октября 1859 года.

«Крайне сожалѣю, любезный Яковъ Ивановичу что вы, какъ кажется, не на шутку занемогли. Убѣдительнѣйше прошу васъ себя поберечь и отложить важныя ваши занятія пока совсѣмъ не оправитесь.

«Обзоръ положенія святаго нашего дѣла и различныя мнѣнія гг. членовъ отъ дворянскихъ Комитетовъ совершенно согласны со всѣми свѣденіями, которыя до меня доходятъ съ различныхъ сто- ронъ. Между тѣмъ кромѣ Шидловскаго Я еще получилъ два адреса отъ 18 и отъ 5 членовъ. Послѣдній въ особенности ни съ чѣмъ не сообразенъ и дерзокъ до крайности. По выздоровленіи вашемъ желаю , чтобы они всѣ были обсужены въ Главномъ Комитетѣ въ Моемъ присутствіи. Если гг. эти думаютъ своими попытками Меня испугать, то они очень ошибаются.

«Я слишкомъ убѣжденъ въ правотѣ возбужденнаго нами святаго дѣла, чтобы кто либо могъ меня остановить въ довершеніи онаго. Но главный вонросъ состоитъ въ томъ, какъ его довершить.

«Въ этомъ, какъ и всегда, надѣюсь на Бога и на помощь тѣхъ, которые, подобно вамъ, добросовѣстно желаютъ этого столь же искренно, какъ Я и видятъ въ этомъ спасеніе и будущее благо Россіи.

«Не унывайте, какъ Я не унываю, хотя часто приходится пере- носить много горя, и будемъ вмѣстѣ молить Бога, чтобы Онъ насъ наставилъ и укрѣпилъ.

«Обнимаю васъ отъ всей души

А.»

I don't read Russian, so I'm having to clean up the line-wrapped words and plug this into Google Translate, which does an acceptable enough job to determine that this is the letter you are looking for.

Then, in the margins of Ya. I. Rostovtsov's letter, the Sovereign Emperor wrote to him with his own hand:

“Gatchina, October 25, 1859.

“I am extremely sorry, dear Yakov Ivanovich, that you, how it seems they are seriously ill. Convincingly, I ask you take care of yourself and postpone your important activities until you completely recover.

"Review of the position of our holy soul and various opinions of members of the noble Committees completely agree with all the information that reaches me from various sides. Besides the one of Shydlovsky I got two more letters from groups of 18 and of 5 members. The latter is wrong and impudent to the extreme. After your recovery, I wish all these letters to be discussed in the Main Committee in My presence. If some members of the Committee think they will manage to frighten Me, then they are utterly mistaken.

“I am too convinced of the rightness of our holy endeavor for anyone to stop me before I am done. But the main question is how to complete it.

“In this, as always, I hope for God and for help of those, who, just like you, honestly desire it just as
sincerely, as I do, and see in this the salvation and future benefits for
Russia.

“Do not be discouraged, as I do not lose heart, although I often have to endure a lot of grief, and together we will pray to God that He guided and strengthened us.

"I embrace you with all my heart.

A."