Optimus is a hard one since he was a data clerk before becoming a Prime I can imagine him having a lot of favorites probably if I had to pick a main favorite for him it would probably be King Arthur and his Knight. Because their both strong leaders and I can see a lot of similarities.

Bumblebee I think would like the Lord Of The Rings series.

Arcee I think would be into the Hunger Games or Divergent series because I can see her relating to Katniss or Tris.

Bulkhead I think would like the Percy Jackson series. (I've never read that one myself but have been meaning to check it out. I watched one of the movies and thought it was good.)

Smokescreen I think would like Ready Player One.

Wheeljack probably wouldn't read very much but I could see him liking one of the H.G. Wells stories like the Time Machine or War Of The Worlds.

Ultra Magnus would probably read a lot of military books or survival manuals. I could also picture him enjoying the Three Musketeers.

Ratchet would probably read a lot of medical books. I also think he might like The Lion The Witch And The Wardrobe. (Don't know why.)

Megatron I think world like Spartacus because he was also a gladiator.

Soundwave would probably like Sherlock Holmes.

Shockwave would probably read a lot of science books and think he might like 1984. ( Again I don't know why. I didn't like that one myself.)

Starscream I think would like The Count Of Monte Cristo. I can also picture him reading The Art Of War, The 48 Laws Of Power and How To Win Friends And Influence People.

Knock Out I could see reading the Kingdom Keepers series. (That's actually one of my personal favorites those book are full of action and suspense which is why I think KO would read them.) Or I can also see KO reading a lot of horror stories like Dr. Jackal and Mr. Hide.

Breakdown I think would also like horror stories like Frankenstein or Dracula.

Predaking I think would like fairytales how funny would that be a predacon who changes into a dragon liking fairytales.

Dreadwing would probably read Ulysses. (I haven't gotten around to reading that one myself I've tried but it hard to understand.)

Arachnid I think might like Charlotte's Web. (Sorry just can't resist making spider and web jokes with her. XD)

The Vehicons would have mixed interests.