If you are serious about learning this field then you should start with http://www.schneier.com/book-applied.html . This is pretty much the industry bible for developers. Please don't make your own routines unless you are doing it purely for learning purposes.
I would suggest you start with learning how e.g the DES algorithm works, and work your way up from there. There is more to be gained from learning all the various encryption mechanisms out there today and knowing what their shortcomings are and how to implement them correctly in a production system, than there is in learning the algorithms in depth. However, if you want to dive deep into the crypto field and don't care so much about implementation in e.g. applications, then do the latter.
XOR obfuscation with single round single byte keys and no salt is insecure. It doesn't take much more than a simple nested for loop to brute force the cleartext. Unfortunately it doesn't stop people using it, however (some NOAA EMWIN satellite decoders use it, for example).
I don't know what you intend to build, but some good off the shelf open source libs include http://libtomcrypt.com/features.html and http://www.cryptopp.com . I've built many apps using libtomcrypt.
It's 'cryptanalysis', btw :)