I stick to ICs from the 1970s and 1980s; after that, things get too hard to understand. Books from that era provide a reasonable description.

For digital circuits, Mead and Conway's Introduction to VLSI Systems is a good source of information. It's available online: https://ai.eecs.umich.edu/people/conway/VLSI/VLSIText/VLSITe... (the link seems to be down right now, though.)

For analog circuits, the book "The Art of Analog Layout" is good if you can find a cheap used copy. The designer of the 555 timer wrote an interesting book "Designing Analog Chips" which I found very interesting. It is available online: http://www.designinganalogchips.com/

There's also an online tutorial on IC reverse engineering: http://siliconzoo.org/tutorial.html