As the title says, I want to read a Japanese history book/s in Japanese as I want to learn how the concept of the samurai was born and developed during Japan's medieval period (while getting my immersion in Japanese for my language studies).
The period that I wish is covered is from the start of the war with the Emishi during the Thirty-Eight Years' War of 773 AD and ends before the onset of the Sengoku period after the Onin war of 1467.
In (relatively) easy Japanese, the following two introductory book might be interesting to you and at least first one (Kawai 2010) ia available also in e-pub like Kindle (just in case OP lives out of Japan):
While the latter is the concise overview of Bushi/ Samurai (though the importance of the archery in the battle of Kamakura Period has been the author's specialty), the former is the classic that re-consider several aspects of the battles in the 12th century.
(Adds): The third standard overview, SEKI Yukihiko, Bushi no Tanjo (The Birth of Bushi/ Samurais) 『武士の誕生』 Tokyo: Kodansha, 2013 (Originally published in 1999), is also balanced and readable book, though it relates rather little on weapons and tactics. This one is also available in e-pub.
If you wish to focus more on the weaponry, Kondo Yoshikazu's books are reputable, but they are not available in e-pub forms (so, as long as you are living in Japan, it is not so difficult to find the copy either in the local library or in secondhand book store.....):
These recommendations are fairly basic ones, but /u/morricane, real expert in this field, might have some more specialized literature to be mentioned.