This is the 75th most frequent Icelandic word.
þessi
The Icelandic word "þessi" means "this" or "these" in English.
In this sentence, 'þessi' functions as a determiner modifying the noun 'bók' (book) and is in the nominative singular feminine case to agree with the noun.
Þessi bók er mjög áhugaverð.
This book is very interesting.
In this example, 'þessir' is the nominative plural masculine form of 'þessi', modifying the noun 'kettir' (cats).
Þessir kettir eru mjög forvitnir.
These cats are very curious.
Here, 'þessa' is the accusative singular feminine form of 'þessi', agreeing with the noun 'mynd' (picture) as it is the object of the verb.