When I'm trying to define what had the greatest influence on my decision to become an illustrator I think back to my childhood. From an early age I was literally surrounded by books about art. My father was (and actually still is) buying tones of such albums (even if some of them are written in languages he doesn't understand).
Personally I prefer books focused on strictly defined topics. This unfortunately means that what is "incredibly valuable" for me, can be "not-oh-so-useful" for others.
Hopefully I haven't discourage you from further reading... Some books greatly influenced my approach to illustration and helped in developing my skills. Here they are:
"Fantasy Art Workshop" brilliantly written by John Howe. The author have not only presented stages of development of his illustrations along with a whole bunch of tips, but he also described his professional path and gave valuable information about freelancing.
"Drawing & Painting Fantasy Landscapes & Cityscapes" by Rob Alexander and Martin McKenna. The subject couldn't fit me better! It's an inspiring set of works created by Rob Alexander and other fantasy artists, enriched with presentation of creating process . Moreover it contains tips on basics in art (composition, color theory, capturing the right mood etc.) and description of elements of landscape and various types of fantasy places (naturally with examples).
"Painting Spectacular Light Effects in Watercolor" by Paul Jackson. This book proves that watercolor works don't have to be delicate, blurry and bright. Quite the contrary, they can be very dark, extremely detailed and hyperrealistic. It changed my attitude to watercolors a lot! The process of creating masterpieces (yes, MASTERPIECES) were presented very clearly, step by step.
"Drawing and Painting the UNDEAD" by Keith Thompson. I bought this book a few years ago as a Christmas gift for my younger sister. I haven't expected then, that it can be so valuable also for me (even though I've never drawn the Undead). What captivated me most was a very well described concept process based on strong logical thinking. The way the author analyzed all the factors combined with his artistic skills contributed to very interesting and inspiring results.
"Naturlära" by Lars Lerin. I suppose that so far the book was published only in Swedish. Anyway hundreds of watercolor works done from nature are just breathtaking. You can find there all aspects of nature: from Swedish winter landscapes to... dead fish.
"Anatomy for the Artist" by Jenő Bartsay - an absolute classic released first in 1953. It contains over 140 pages filled with very accurate and aesthetic sketches of bones, muscles and other parts of a human body. I haven't done all the exercises yet, frankly speaking I'm not even close to the end, but I'm not going to give up!
And most of all I'd like to recommend countless albums about:
-renaissance artists (and their technical approach to artworks),
-baroque artists (and their extraordinary focus on chiaroscuro),
-landscape paintings form 19th century (and their amazing atmosphere).
As you can see, there is none strictly about perspective among them. If any of you encountered such a book, let me know! Or maybe you know other good books about art?