I've often read the tip of refreshing a merchants inventory: Buy the cheap ammo and rest for 24h.
So I tried this with Anna (in Arnika). After resting she had replaced all the ammo I had bought with loads and loads more ammo. No new armor or misc items. When I tried it again it became even worse. The only thing she sells at the moment is weapons, primarily AMMO.

Any way to remedy this?

first, what do you actually want to buy? if you want to buy armor, then buy armor from the merchant and wait for the merchant to refresh his armor in the inventory. if weapon, then buy weapon, if ammo, then buy ammo. etc.

second, I believe different merchant will sell different things. try other merchant. The rapax ( forgot his name ) in arnika sell weapon.

I was hoping to find some spellbooks, especially low-level alchemist spells.

I didn't know you had to buy the same category of stuff you wanted refreshed. The problem is that Anna does not sell any armor or misc items (including spellbooks) anymore. Only weapons.

Antone (the rapax in Arnika) sells weapons and armor.

Buying cheap ammo is only necessary if the shop inventory is full. That happens only very rarely. To throw in a number, i once bought more than 10000 bullet stones from a merchent, and i don't think his list was full before.

Anna only sells a small number of different books, mostly psionic stuff IIRC. I think for alchemist books Kunar (at Umpani Base) is better.

Shops restock three times a day. But it seems to me that the items they refresh are differently for these three restocks. At least when i wait for Kunar to restock books (i usually make a shopping trip to him before i go to trynton), it seems he only stocks new books once a day (together with other items). And i think once a day he only restocks ammo.

try crock in the swamp for good selection of spellbooks ;)

Crock only has level 4 spells (but i think all of them).
Your party has to be level ~11 to get them.

Since the original poster wanted low-level alchemist spells: Sadok has many of them.

Thanks for the help. I found the books I needed.

There is on more thing I noticed with Anna. When you sell an item to a merchant it will appear in their inventory. Even when it is a type they normally do not sell. With Anna, this doesn't happen anymore. All her miscelaneous and armor items have vanished and when I sell some armor it vanishes too.