From everything I've heard they have improved (I never used the older version, so I can't give an actual comparison but the current version's security is fairly decent). What exactly are you looking for in security features? Each compiler has slightly different methods for protecting your book, but I don't think you can get it 100% theft-proof.
The best suggestion I can give you is to make a list of what security issues concern you and go from there (another thing that plays a part is securing your download page). Once you decide what features you would like, then find out if they are feasible and if the benefit is worth it. For instance, there is an exe compiler that allows you to make the book available only on the users computer (it's coded for their computer and won't run on any other computer); however there is a lot of extra work involved - you have to compile each copy of the book seperately for each user plus it doesn't allow instant access after purchase. This method also could alienate customers (what if the want to use it on both their home and business computer, get a new computer, etc). You would have to decide if the benefit of securing the book this way is worth it.
If I can help you further, let me know