Learn Italian: Ho già letto la recensione: il film è bellissimo. - I have already read the review: the movie is very beautiful.

It comes from the adjective bello (beautiful). To make it mean 'very beautiful' or 'extremely beautiful' (the absolute superlative), you drop the final vowel -o and add the suffix -issimo.

Does the ending of bellissimo change depending on what it describes?

Yes! Just like a regular adjective, it has to agree in gender and number with the noun it describes. Since il film is masculine and singular, the adjective is bellissimo. If it were describing a feminine noun like la recensione (the review), it would be bellissima.

Why is già (already) placed right in the middle of the verb?

In Italian, common short adverbs of time like già (already), sempre (always), and mai (never) are typically placed right between the helping verb (ho) and the past participle (letto) in the past tense.

Why doesn't the past tense of leggere (to read) end in -uto?

The verb leggere has an irregular past participle. Instead of following the standard rules to become something like 'leggiuto', it changes to letto. You just have to memorize this one!