Breakdown of Wczoraj przeczytałem ciekawą książkę.
książka
book
ciekawy
interesting
wczoraj
yesterday
przeczytać
to read (perfective)
Questions & Answers about Wczoraj przeczytałem ciekawą książkę.
Why use przeczytałem instead of czytałem?
This is the main point of perfective verbs! You use przeczytałem to show that you completely finished reading the book. If you used the imperfective verb czytałem, it would mean you were in the process of reading it yesterday, but leaves it open whether you actually finished it or not.
Does the ending of przeczytałem tell us who is speaking?
Yes! The -łem ending shows that the speaker is a man. If a woman were saying this sentence, she would use the feminine past tense form and say przeczytałam.
Why is it ciekawą książkę instead of the dictionary forms ciekawa książka?
The book is the direct object of the verb (it is the thing being read), so it needs the Accusative case. For feminine words, the noun ending -a changes to -ę (książkę), and the adjective ending -a changes to -ą (ciekawą).
Why is there no word for "I" (ja) in this sentence?
In Polish, the verb ending already does the job. The -em at the end of przeczytałem tells us exactly who did the reading. You can drop the pronoun ja entirely unless you want to emphasize that you were the one who read it.