There's a lot going on at the moment.
Something just came over you.
He gave you gifts bought for his ex.
You were horrified.
You're not sure you know him anymore.
OK. Let's leave the 'bad Owl snooping' recriminations aside for a moment.
Something prompted you to take a look. Your trigger was the glimpse of past correspondence with previous partners, but in a normally relaxed relationship you'd have been able to mention them, and/or have already known everything about those relationships. As things are, neither of you said anything and he deleted them straight after. This obviously doesn't mean they're still in a relationship, but it does indicate very high levels of privacy, for want of a better word. Many people would be okay with this. I wouldn't.
I'd be interested to know what horrified you about his letters. It looks very much as if they're more intense, emotionally and sexually, than anything he's ever sent to you. Am I right, or is it something else, or like that but more as well?
Whatever this was, it's left you feeling you don't 'know' him. That usually means you know the person your partner has presented himself to be, with whom you fell in love, but now you've discovered whole other sides to his character. Even if you don't dislike these newly-discovered aspects of a partner, it's incredibly disconcerting and certainly shows you've not been in a relationship with the whole person, as it were. It's upsetting.
The recycled gifts would piss me off massively. Not so much if he'd told me why he had them, along the lines of "would you like this now?" but I'd be as cheesed off as hell to find he'd simply bunged me some stuff labelled "girlfriend presents" from the back of his wardrobe! It feels very shabby to me - cheap and dishonest. I'd rather get a box of MY favourite biscuits than a bottle of some other woman's favourite perfume.
This all leads us to ... What's the "a lot going on"? Are you feeling distanced from him; has he been acting in ways that seem out of character? Maybe your unconscious has added up a bunch of clues which hadn't quite made its way to the front of your mind yet.