ToME: the Tales of Maj'Eyal

If a lore has been already placed in one of the levels I have been to but found too dangerous or annoying(some dark narrow mazes filled with traps, for example) to fully explore, is it possible that I can get the lore in somewhere else? Or if I have to fully explore every level later if I really want to collect them?

There's no gain without pain I'm afraid. If you like the lore that much then go get it ;)

However you only need to get any lore piece once to add it to your global found lore list, and then all characters you ever generate will have that lore when they find the super special library feature.