Question

the pural names for books and countries ends with '-s'  but they are used in singular Why?

Answer 1

These types of nouns do not have plural forms. ... 1) “A” and “An” are used withsingular countable nouns that are non-specific ... Explanation: “Book” here is asingular countable noun but is NOT one ... English is popular in many countries

Answer 2

English nouns are inflected for grammatical number, meaning that if they are of the countable type, they generally have different forms for singular and plural. This article discusses the variety of ways in which English plural nouns are formed from the corresponding singular forms, as well as various issues concerning the usage of singulars and plurals in English. For plurals of pronouns, see English personal pronouns.