I was predicted 3 As and got an offer from UCL of ABB, but lots of strong candidates with similarly high predictions did seem to get rejected this year.
They seem to base the application process quite heavily on the PS, and then give offers to most (I think about 2/3) candidates who are invited to interview.
Because of the emphasis on the PS/ interview, I'd guess that predicted grades are not as important as they might be elsewhere; as long as you seem likely to meet their standard offers of ABB/AAB, I think it's unlikely they'll reject you based purely on them. (Also bear in mind that most schools are more likely to be optimistic with your predicted grades, so unless you know for sure otherwise, you may be predicted 3As).
Books-wise, I would say Gombrich (Story of Art) would be perfect for you, as you say you don't have an in-depth knowledge yet - it should fill in some blanks reasonably well. Other than that, I'd say it's most important to read books that cover your interests within the field, because you'll be able to talk about them more passionately in interview. You don't necessarily need to have read a load of stuff when you write the PS, as long as it's informed and enthusiastic, but it definitely helps to have read up about what you claim to be interested in before your interview, so you can qualify it. I would also recommend Kenneth Clarke's 'Civilization,' and 'Ways of Seeing' by John Berger for general reading (or try and find the original TV series for both).
The interviews seem to vary quite a lot, but mine was very chilled out and centred around what I'm interested in, rather than grilling me to try and identify what I didn't know! It does seem more important to know what you're into and be able to explain why, rather than just knowing heaps and heaps of stuff. They may also ask you to analyse a copy of a work of art that they show you, so it's worth practising for this by opening an art book at a random page and trying to talk intelligently about whatever you come across.
As to your subjects, they sound fine to me... most universities I applied to expressed a preference for humanities, and history in particular is obviously helpful.
Hope that helps! Good luck.