Normally the answer to this is a "we can't say" as already indicated in the Comments, however, with the intended use I think one can say yes in this case. Given that the manufacturers of treated exterior woods like this say that their product is OK for things like garden benches and tables where there is occasional bare-skin contact — bearing in mind the highly litigious age we live in — just incidental contact indoors must surely be safe. And after planing off the surface any risk goes down substantially.
There's a but though. The major risk here is to the user, i.e., you, while you're working on the stuff. While I wouldn't lose any sleep over occasional handling of such material with bare hands and working with it in the shop a little bit without taking any special precautions1, I'd be very hesitant to do much machine processing2 without suitable PPE.
Your call
No matter whether you read up further on the subject of the safety of dealing with treated lumber, at the end of the day this is going to come down to an individual call. Just like many things in woodworking (use of crown guards on table saws, always wearing a mask when sanding or only sometimes, laying routers on their side while they spin down etc. etc.) right or wrong everyone makes their own call each time, based on essentially gut feeling. So what's your gut telling you?
If not, can I somehow seal the wood? Preferably without loosing the natural color/grain or making the planks look very glossy (I really don't like the look of a poly-urethane coating).
Polyurethane isn't one thing. For starters we now have both oil-based and waterbased finishes that contain polyurethane and even with the same gloss level they typically give a very different look to wood based only on their native colouring — oil-based being a classic yellowing/amber tone and waterbased usually being colourless (milky in the tin, but dries water-clear).
In addition to this you can apply even a full-gloss poly in dilute form (wiping varnish) which if not built up in many layers gives a very different look to the classic varnished finish. Using just a couple/few light coats you get very much an in-the-wood look.
But that aside, almost the standard recommendation these days to "What can I put on wood to provide protection while changing the look as little as possible?" is a matt waterbased poly. At best these literally make wood appear as if it has nothing whatever on the surface. So if you wanted to give the wood as much protection as possible without any/much alteration of the look this is the thing to go with.
However bookshelves don't typically need much protection as they not handled much, they're not moved regularly and they usually aren't somewhere a sweating glass or hot mug of tea/coffee will be placed. So if you wanted to just wax them that would likely be fine; many people have light-use interior projects finished with only wax and are happy with how they hold up over time. Note: prior to waxing it's quite common to use sanding sealer or a light coat of a dilute finish such as some diluted shellac, or wiping varnish. In addition to some added protection and maybe an improvement in looks this also makes it easier to build up a consistent finish compared to the wax going on to bare wood.
1 Many user-made indoor workbenches feature pressure-treated wood for the legs and possibly other components as well, and reports of e.g. skin irritation are rare.
2 Chiefly planing/thicknessing, but also any sanding.
