Tafsir of Al-Isra 17:9

Indeed, this Quran (which We have given to you; this relates to the beginning of the Surah, as has already been indicated. The use of "this" here serves to glorify that which was brought by the chosen Prophet, may Allah bless him and grant him peace) guides (that is, all of mankind, not a specific sect among them, unlike the custom of the Book which We gave to Moses, peace be upon him) to that which (i.e., the way which) is most upright (meaning the most upright and correct of paths, I mean the religion of Islam and Monotheism. "To that which" is an adjective for a noun omitted for brevity; some estimated it to be "the state" or "the religion." Whichever you estimate, you will not find with the explicit statement the rhetorical flavor that you find with the omission, due to the indication within this ambiguity that it has flooded the valley and covered the villages. "Most upright" is a superlative, as indicated by more than one scholar).

Abu Hayyan said: "What appears from the perspective of meaning is that the superlative is not intended, for there is no sharing between the path to which the Quran guides and other paths in the root of the derivation such that it could be superior to them. Thus, the meaning is: 'to that which is qayyimah,' meaning upright, as Allah the Almighty said: 'In which are upright writings' and 'that is the religion of the upright.' " Imam al-Razi held this same view.

And gives good tidings to the believers (by means of the rulings and laws contained within it).

Abdullah, Talha, Ibn Waththab, and the Kufans recited "yubshiru" (gives good tidings) in the light form, which is the present tense of bashara. Both bashartuhu and abshartuhu are used.

Who do (the deeds) righteous deeds (which have been explained therein) that they (i.e., that they have, in recompense for their deeds) will have a great reward (in terms of essence, and in terms of multiplication, tenfold and beyond). Ibn Jurayj interpreted "the great reward," as well as "the generous provision" throughout the entire Quran, as Paradise.