The possessive for a singular book in "your book" is "er bok" and for plural books in "your books" is "era böcker." https://en.wiktionary.org/wiki/bok#Swedish (Click on declension. This shows all forms of "bok". Thank you Arnauti, for noticing that I had put the Norwegian link and plural, so now it is fixed back to Swedish. I must have been asleep!)
Scroll down on this page for all of the possessive pronoun forms: https://www.duolingo.com/skill/sv/Possessives
"din" is the form used for "your" when "you" is singular as in "du". "din bok" is for one book of one person who is you, while "dina böcker" is for plural books of one person who is you. "ditt" is the form for singular neuter nouns.
"er" is the form used for "your" when "you" is for more than one person as in "ni". So the pronouns show singular or plural "you" and also show singular common, singular neuter or either plural books.
"er" is the form for describing singular common nouns, "ert" is the form for describing singular neuter nouns and "era" is for describing plural nouns of plural people who are you. http://www.readersstuffz.com/downloads/ebooks/Language%20Books/Swedish/Swedish%20-%20Essential%20Grammar.pdf
If you were only given the sentence ”It is your book” you wouldn’t know in English whether you’re talking to one person or to many (you all’s book), therefore when translating from English to Swedish, both din and er is acceptable, but you have to know that din is only when talking to one person and er is to many people.
'This' is used to refer to something close to the speaker. It can be translated into Swedish as den här or denna (or det här and detta). 'That' is used to refer to something that is farther away from the speaker. It can be translated as det där or den där and in some cases as det and den.
If you know Polish (looking at your username) it's a bit like 'ten' vs 'tamten' although we don't always use them for the same things – I think 'ten' is used much more in Polish than 'det här' and 'this' in Swedish and English.
Din, ditt, and dina are all used when the "you" is a du, i.e. only one person. Din is used with singular words that take en, ditt is used with singular words that take ett, and dina is used with plurals.
Er, ert, and era are all used when the "you" is a ni, i.e. more than one person. Er is used with singular words that take en, ert is used with singular words that take ett, and era is used with plurals.
I could nevwr figure out the difference between ni and du until i got here and had trouble with dina and ert. Reading through the comments and being able to ask other people questions really helps clear things up.
It would be nice if duolingo would do a bit of explaining when introducing things like this but the comment section makes up for it. Thank you to all the users that help confused people like me! This app would not be effective without you!