A-level Latin Watch

I just sat Latin, and I would first mention the fact that this is a rather new specification. I sat the first year of the paper which probably made it a little harder, but you will be able to work from our past papers, mark schemes and examiners' reports.

For Latin language: John Taylor's Latin Beyond GCSE is pretty much the go-to. Personally, I'd say work through all of that before looking at Owens' books. But Owens' books were extremely helpful and I highly recommend them.
In general, Latin language is just vocab, grammar and translation practice. For the vocab, you need to learn it extensively! I would suggest: OCR AS vocab list (learn inside out and back to front), then learn the entirety of John Taylor's vocab list at the back of Latin Beyond GCSE, and also Taylor's "300 Verse Unseen words" because that's extremely helpful for the verse unseen. I also suggest looking at AQA's old vocab list for its now-scrapped Latin A level and the extra vocab lists at the back of Owens' Prose and Verse unseens books - if you know ALL of that vocabulary, you will be absolutely fine! Don't forget to learn principle parts for verbs, that's extremely important!
For grammar, John Taylor is pretty good, but I had to supplement it with a lot of other worksheets from school and online resources and I then compiled my own grammar guide. I would suggest learning all accidence (tables) off by heart and then practicing all the syntax (like indirect statement etc) in Taylor's book. I haven't used any additional Latin grammar books, but I reckon they would be helpful since Taylor wasn't fantastic.
Translation practice is as simple as working through all the John Taylor practices, followed by the old exam specification language papers, followed by as many of Owens' translation as you can manage.

Doing all of that will pretty much sort you out for latin language unseens and comprehension - for composition you will need to get a composition textbook (I can't recommend one since my school gave me their own handmade one) and: learn all the vocabulary english-->latin, learn all the genders/parts of nouns/adjectives and practice all of the syntax english to latin meticulously. Personally, I thought that the amount of work needed for composition was unnecessary since improving my translation skills so that I could ace the comprehension massively improved by unseen translation skills.

For latin literature, the ZigZag guides are amazing - your school will need to buy a license, since they are quite expensive. They are a thorough guide to the texts. Terry Bird also creates a really great latin text version with running vocabulary and translation help. I would use online mark schemes to your advantage as the marking seems to be really weird and difficult to understand for literature.

Hope this helps!