I am a high school student but never observed math in such a way where I could feel it just applied it wherever required. Nowadays going through a lot of phenomenons I have started to feel what do I know about maths. Looking back I got to notice that I literally know nothing about it. My fundamental knowledge of mathematics is even lower than a 4th grader. I have been looking through many sites about books to start with from scratch but became confused. I hope that this forum could suggest me a constructive list of books to start with. I might have missed answers regarding this in this forum.
However I am requesting to thos who have atleast once seen this question to suggest some books which will be good to start mathematics from the very beginning.
In answer to your question, I will suggest some books for grades 1 to 6 below which are mathematically sound and can be read online.
However, I'd like to point out first that while it is very common for adults or high school students to feel that they have gaps in their understanding of elementary mathematics, it's unusual that the best thing for them would be to read books truly at the level of children learning the material for the first time.
More commonly, it would be best to use a book for adults such as Teach Yourself Mathematics by Neill that seeks to fill gaps adults typically have, or else a textbook intended for preservice elementary teachers such as Mathematics for Elementary Teachers by Beckmann.
But here are some school books going from grade 1 to grade 6 that can be read on the Internet Archive, the Elementary School Mathematics series by Eicholz and O'Daffer.
Grade 1 pupil's book
Grade 1 teacher's book
Grade 2 pupil's book
Grade 2 teacher's book
Grade 3 pupil's book
Grade 3 teacher's book
Grade 4 pupil's book
Grade 4 teacher's book
Grade 5 pupil's book
Grade 5 teacher's book
Grade 6 pupil's book
Grade 6 teacher's book