This book has been, without a doubt, crucial in aiding my understanding of cryptosystems and why things are the way they are, and how do these cryptic crypto algorithms even work. If you are interested in learning how to develop software that are 'correct' and secure, then this is a great book to understand what are the primitives of information security, what algorithms already exist and which ones to use in what scenario.
So the motivation to pursue a thorough understanding of cryptography and to gain the ability and knowledge required to make a secure cryptosystem came sometime after college ended, when I and Kunal were working on a terminal chat application that would support end-to-end encryption. At that time, I hardly knew what I had gotten myself into (which is similar to a lot of things in my life), as the application development part seemed very simple. We got done with the application part, terminal app and the backend, and then came the encryption part, and that is when the knowledge about existing techniques and understanding of basic crypto primitives fell short. And that is when I started reading about cryptography and stumbled upon this book.
Although they seemed daunting at first, both the books are very accommodating for a wide range of audience, right from someone like me who barely knew what a block cipher is, to the more experienced folks who might understand all of the mathematics given in the book in the first go. While not very complex (school grade algebra with addition, multiplication, modulus and xor operations), it takes a little effort (read: re-reading a topic 3 times, sometimes more) to actually get what's happening, why an operation is being performed, for example.
While reading the first book, remember that it was written when I was literally a year old, in 1996. Hence, although the engineering principles and general recommendation is still valid, you need to keep in mind that the algorithms recommended in that book are not valid (as attacks are found for many of them and DES has officially retired), and that is corrected in the second edition of the book. In any case, studying the DES algorithm in detail should be a delight for any crypto nerd, regardless of its practical value.
The second version is more up to date, and for some reason I was more comfortable reading it than the first one. It might be because I knew a little more while reading the second edition, which can be a good tip: If you're serious about understanding cryptography from an engineering standpoint, skim over the first book and make a note of everything that you find useful and interesting, and do a more detailed study of the second edition of the book.
What I found nice about the books is, they really are 'applied' books. It isn't all mathematics and algorithms, but the actual merger of these algorithms into real world systems. In the real world, cryptography and cryptosystems don't exist in isolation, but play a small role in the larger scheme of things. Breaking a cryptosystem is usually reserved for the more resourceful adversary, and while these (well established and peer reviewed) cryptographic primitives rarely fail, when they do, it is catastrophic. The computational infeasibility makes the theoretical aspect of cryptography very secure. Problems appear when they are implemented, and that is where the bugs start to show up. Then there is the software development methodology which usually prioritises deadlines and features above security. There is a section dedicated to explaining what 'trust' is, how it forms such an important aspect of information security and secure software development. Overall, the book is quite interesting to read, and the content is without a doubt top quality, which is what one expects from Schneier.
In closing, I'd recommend this book if you are into security and wouldn't mind knowing the details of some of the fundamental algorithms that make the digital revolution possible. Thank you for reading.