Translation:I bought two thick books and one thin book.
There are three places to put counters.
After the particle: 「本を六冊買った」、"I bought six books"
After the noun: 「厚い本三冊と薄い本五冊を買った」、"I bought three thick books and five thin"
Before the noun: 「二冊の本を買った」、"I bought two books".
So what's the difference between the three? #3 should only be used for emphasis. Say someone asks, "how many books did you buy again?", then #3 is the appropriate response.
#1 is the most common, but for this sentence, you have to use #2, because 「厚い本と薄い本を二冊一冊買った」、doesn't work.
More reading on the topic below:
This is a great example as to why people ask for more kanji. That being said it needs a separate mode or more flexible and accurate hints since at this point I can't learn from the current use of kanji in Duolingo either - I'm trying to learn vocab first and already I read a bunch of kanji that I can translate but don't know how to pronounce because their hiragana is only rarely mentioned in the comments or not mentioned at all.
Being pedantic has its place in language learning. In casual speech, "a thin book" and "one thing book" are interchangeable. For studying language, they have clear differences. "One book" places emphasis on the amount you bought, clarifying that you did not buy any more than one, whereas "a book" does not draw focus to that detail at all & the descriptive focus instead goes to how the book is thin. Similar to "ha" and "ga" in japanese. Having the ability to emphasize certain parts of speech with a specific particle like "ga", or in this case, in english, with a specific numerical preposition rather than a generic indefinite one, enriches the nuance and ability of language.
Might just be me and my weird brand of Washington English, but "a" doesn't seem to be a generic indefinite preposition to me. In other words, there doesn't seem to be any difference between "a" and "one" when used as a number.
"One hundred" versus "a hundred"
"One thousand" versus "a thousand"
Notice how "a" cannot generally precede or be used to replace the number when the number is something other than one. Some incorrect examples to follow.
"A two hundred"
"Those a hundred spartans (in reference to 300 spartans)"
"A two men approached me"
<sub>~</sub>Now for some other examples involving men
"Men approached me." (Number is undefined)
"A man approached me." (One man)
"One man approached me." (One man)
"Man approached me." (Sounds weird, needs something before man like the word "the")
"A men approached me." (Sounds weird here too as "a" [singular] is used with "men" [plural])
You may be gramatically correct with that using the word "one" focuses on the amount bought moreso than by using "a", but there gramatically can only be one and only one book if the word "a" is used. Therefore, the sentence shouldn't be incorrect on the basis that saying "one thin book" is more correct than "a thin book". It seems that Duo accepts the use of "a thin book" as of 2018/8/5.