Are the aforementioned books approved for issuing fatwas according to the Shafi'i school of thought, and is it permissible to take from them?

The books mentioned are from the Shafi'i school, and the two relied upon by later scholars for understanding the madhhab are: Tuhfat al-Muhtaj and Nihayat al-Muhtaj.

All the mentioned books are reliable and can be depended upon, but with consideration given to prioritizing some over others. It is permissible to act upon all of them for oneself. However, for issuing fatwas, when there is a difference of opinion, al-Tuhfa and al-Nihaya are preferred. If they differ, the mufti may choose between them if he is not qualified to make a preference. If he is qualified, he should issue a fatwa based on what is more preponderant.