Beshalach #3: Write it in the book. Which Book?

Towards the end of Beshalach, after we are told of Yehoshua's victory over Amalek, Moshe is told to write something in "the book." Shemot 17:8-16:

ח וַיָּבֹא, עֲמָלֵק; וַיִּלָּחֶם עִם-יִשְׂרָאֵל, בִּרְפִידִם.	8 Then came Amalek, and fought with Israel in Rephidim.
ט וַיֹּאמֶר מֹשֶׁה אֶל-יְהוֹשֻׁעַ בְּחַר-לָנוּ אֲנָשִׁים, וְצֵא הִלָּחֵם בַּעֲמָלֵק; מָחָר, אָנֹכִי נִצָּב עַל-רֹאשׁ הַגִּבְעָה, וּמַטֵּה הָאֱלֹקִים, בְּיָדִי.	9 And Moses said unto Joshua: 'Choose us out men, and go out, fight with Amalek; tomorrow I will stand on the top of the hill with the rod of God in my hand.'
י וַיַּעַשׂ יְהוֹשֻׁעַ, כַּאֲשֶׁר אָמַר-לוֹ מֹשֶׁה--לְהִלָּחֵם, בַּעֲמָלֵק; וּמֹשֶׁה אַהֲרֹן וְחוּר, עָלוּ רֹאשׁ הַגִּבְעָה.	10 So Joshua did as Moses had said to him, and fought with Amalek; and Moses, Aaron, and Hur went up to the top of the hill.
יא וְהָיָה, כַּאֲשֶׁר יָרִים מֹשֶׁה יָדוֹ--וְגָבַר יִשְׂרָאֵל; וְכַאֲשֶׁר יָנִיחַ יָדוֹ, וְגָבַר עֲמָלֵק.	11 And it came to pass, when Moses held up his hand, that Israel prevailed; and when he let down his hand, Amalek prevailed.
יב וִידֵי מֹשֶׁה כְּבֵדִים, וַיִּקְחוּ-אֶבֶן וַיָּשִׂימוּ תַחְתָּיו וַיֵּשֶׁב עָלֶיהָ; וְאַהֲרֹן וְחוּר תָּמְכוּ בְיָדָיו, מִזֶּה אֶחָד וּמִזֶּה אֶחָד, וַיְהִי יָדָיו אֱמוּנָה, עַד-בֹּא הַשָּׁמֶשׁ.	12 But Moses' hands were heavy; and they took a stone, and put it under him, and he sat thereon; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; and his hands were steady until the going down of the sun.
יג וַיַּחֲלֹשׁ יְהוֹשֻׁעַ אֶת-עֲמָלֵק וְאֶת-עַמּוֹ, לְפִי-חָרֶב. {פ}	13 And Joshua discomfited Amalek and his people with the edge of the sword. {P}
יד וַיֹּאמֶר ה אֶל-מֹשֶׁה, כְּתֹב זֹאת זִכָּרוֹן בַּסֵּפֶר, וְשִׂים, בְּאָזְנֵי יְהוֹשֻׁעַ: כִּי-מָחֹה אֶמְחֶה אֶת-זֵכֶר עֲמָלֵק, מִתַּחַת הַשָּׁמָיִם.	14 And the LORD said unto Moses: 'Write this for a memorial in the book, and rehearse it in the ears of Joshua: for I will utterly blot out the remembrance of Amalek from under heaven.'
טו וַיִּבֶן מֹשֶׁה, מִזְבֵּחַ; וַיִּקְרָא שְׁמוֹ, ה נִסִּי.	15 And Moses built an altar, and called the name of it Adonai-nissi.
טז וַיֹּאמֶר, כִּי-יָד עַל-כֵּס קָהּ, מִלְחָמָה לַיהוָה, בַּעֲמָלֵק--מִדֹּר, דֹּר. {פ}	16 And he said: 'The hand upon the throne of the LORD: the LORD will have war with Amalek from generation to generation.' {P}

What is this sefer, book, mentioned in pasuk 14?

Perhaps it is equivalent to the phrase "mark my words." That is, Hashem is making it clear that this will happen. This could be the meaning even if the intent was to write it in an actual sefer.

Ibn Ezra claims that this command happened in the 40th year in the desert, the proof being the patach under the bet in basefer. It means the book, which would refer to the sefer Torah, written in the 40th year. (I think this means the command but not the war. It makes sense that this is just the command to write it, for as we read in pasuk 8, Amalek fought with Israel in Refidim, which was a fairly early encampment. Further, parshat Zachor speaks about Amalek attacking when they came out of Egypt. So, just the command. Indeed, there is break in the Torah between the war and the commandment, marked by פ.)

What, then, is referred to by sefer? It could be Devarim 25:17-19, in parshat Ki Teitzei:

יז זָכוֹר, אֵת אֲשֶׁר-עָשָׂה לְךָ עֲמָלֵק, בַּדֶּרֶךְ, בְּצֵאתְכֶם מִמִּצְרָיִם.	17 Remember what Amalek did unto thee by the way as ye came forth out of Egypt;
יח אֲשֶׁר קָרְךָ בַּדֶּרֶךְ, וַיְזַנֵּב בְּךָ כָּל-הַנֶּחֱשָׁלִים אַחֲרֶיךָ--וְאַתָּה, עָיֵף וְיָגֵעַ; וְלֹא יָרֵא, אֱלֹהִים.	18 how he met thee by the way, and smote the hindmost of thee, all that were enfeebled in thy rear, when thou wast faint and weary; and he feared not God.
יט וְהָיָה בְּהָנִיחַ ה אֱלֹהֶיךָ לְךָ מִכָּל-אֹיְבֶיךָ מִסָּבִיב, בָּאָרֶץ אֲשֶׁר ה-אֱלֹהֶיךָ נֹתֵן לְךָ נַחֲלָה לְרִשְׁתָּהּ--תִּמְחֶה אֶת-זֵכֶר עֲמָלֵק, מִתַּחַת הַשָּׁמָיִם; לֹא, תִּשְׁכָּח. {פ}	19 Therefore it shall be, when the LORD thy God hath given thee rest from all thine enemies round about, in the land which the LORD thy God giveth thee for an inheritance to possess it, that thou shalt blot out the remembrance of Amalek from under heaven; thou shalt not forget.

where there is a command in the extra sefer Moshe wrote, Devarim, where remembering to destroy Amalek is mentioned.

I would suggest as an alternative that the pasuk is self referential. "Write it in a sefer!" And Moshe did so, by writing the command in sefer Shemot. Indeed, there is one opinion that the Torah was given on an ongoing basis and written in scrolls, combined at the end into the Torah as we have it today. If so, it could be Shemot. But if it is Shemot, it need not be (and it makes sense that it would not be) in the 40th year.

to be continued... (yes, I know it is parshat Yitro - I have a dvar for it I'll post shortly.)