Sorry if this isn't what you're looking for. I just read your other reply that you're looking for philosophical logic. I've got no experience with that at all and can't give a suggestion. But good luck with your learning!
This wasn't my introduction to logic, but I like to skim it for refreshers, and it seems well-written enough to me.
My introduction was What is Mathematical Logic? by Crossley et al., which is a master class in terseness and probably more suitable for someone who's already got a foundation in logic. I must've read the first three chapters three times each, and the rest fewer times than I should've :-).
> It's quite a bit more efficient to take more courses at once, especially if they're closely related (e.g., topology, abstract algebra, real analysis, ...).
I agree that the interplay between related topics helps me form a more robust of understanding of the material.
On the other hand, it might be worth considering proactive and retroactive interference, (the difficulty of storing similar, long-term memories).
The layman's takeaway is that it's generally better to learn a variety of non-related topics concurrently instead of similar ones in order to facilitate better long-term recall.