I have now a good library of math pdf books on my iPad. I use GoodReader app, that offers wonderful features like cropping the pages, so they fit well on the screen and you haven't to zoom in and out all the time, and I read a lot of PDF with that.

For a good experience, the PDF must have already inside the table of contents, so you can jump easily from a chapter to another, but often pdfs don't have it, so I have to add it to them manually with adobe acrobat.