I'm an artist and I wanted to illustrate some of my fav poems into a book. The poems are mostly from before 1900. Is it alright to illustrate them? and meanwhile we're at it, is it alright if the compilation is published?
Is it alright to illustrate them?
Sure. I don't see why it wouldn't be.
Is it alright if the compilation is published?
This is harder to answer. The poems from before 1900 will all be in the public domain by now, so you should be able to re-publish them without any sort of copyright issues. You'll have to check each of the remaining poems and, if they're still under copyright, you'll have to either omit them from the book or go without publishing it.
Copyrights last for the life of the author and for 70 years after that. With works that are commissioned by an individual or organization, the copyright lasts for 95 years after the work was first published. If not published it can last for 120 years after its creation. Many poems are in the public domain. This means they can be used by anyone.