Learn Turkish: Bu tarih kitabı çok sıkıcıymış. - This history book was apparently very boring.

The adjective is sıkıcı (boring). Because it ends in a vowel, we must use the buffer letter y before adding the inferential suffix -mış (apparently was).

Does the speaker know firsthand that the book is boring?

No, that is the exact purpose of the -mış suffix here! It shows that the speaker heard this information from someone else or read a review. If they had read the book themselves, they would use the direct past tense and say sıkıcıydı (it was boring).

Why does the sentence use kitabı instead of just kitap?

Tarih kitabı (history book) is a noun compound. In Turkish, when two nouns are combined so that the first describes the category of the second, the second noun gets a possessive suffix. So, kitap gets the suffix -ı (and the hard p softens to b because of the vowel).

Where does the word sıkıcı come from?

It comes from the verb sıkmak (to bore / to squeeze). Adding the suffix -ıcı creates an adjective that describes something causing that action. So, something that causes boredom is sıkıcı.