Wendell Odom's CCNA books are here (it's a 2 book set):
Don't worry about the other Cisco books right now; you can chase after those once you've covered the basics. Since you're not in IT now, I'll mention that there would be a lot of value to you personally to get CCNA certified and try to use that to get into a networking position so that you can start building your skills on the job. You can move into security from there; most companies will want you to have a networking/sysadmin background if you don't already have security experience; they don't typically hire straight into a security role.
TCP/IP Illustrated Vol I. does not cover pen testing. It covers a little bit of security (in the second edition) but only as it relates to protocols like IPsec. It does cover traffic analysis and will give you most of the background you need to develop that skill. Some of the other things I mentioned (e.g. OS Identification) are covered in pentesting books, but others aren't. There are several articles about port scanning and OS Identification in Phrack magazine (www.phrack.com). The classic paper on IDS evasion is here (http://insecure.org/stf/secnet_ids/secnet_ids.html) but it's dated. I don't know of an up-to-date paper on the topic.
With regards to exploits: you need to be able to modify tools and exploits for various reasons. Sometimes a tool won't compile, other times you want it to do something slightly different. You also need to be able to write small programs/scripts to automate tasks, parse logs, etc. For web applications, you need to be able to exploit vulnerabilities for cross-site scripting (XSS), cross-site request forgery (CSRF) and SQL injection without a canned exploit. You have to understand those exploits and while you may have some cut-and-paste code snippets that you use, you'll modify them and create your own variations as well. For other types of vulnerabilities such as buffer overflows, you don't need to be able to write your own exploits; those take time to create and I can't imagine your clients will want to pay you for that.