The Definitive Guide to Data Quality
A Talend ETL job is simply a Java application. Depending on how you implement the system, you can protect how you built the job....as far as making it very difficult for someone to figure it out unless they decompile and hack around with the revealed code. With the Enterprise Edition, you will usually have your source code available on a DEV system. Up until recently, you needed to leave the source code with the installation so that it could be compiled every time the job was deployed. However now (v5.6.1+) you can simply leave your customer with the compiled artefact in an artefact repository (Nexus by default).
I would question why you would want to do this though. I don't know of many customers who would be happy with a black box integration system where they have no option of working on it without going back to the original team of developers.