Drupal books are a great way to organize content. Unfortunately, there is no way to control access to individual books by default. Like default forums, it's all or nothing.
PHP is an interpreted language. This means that each time a PHP generated page is requested, the server must read in the various files needed and "compile" them into something the machine can understand (opcode).
Clients seem to come in sets. A couple months back, I had several clients who had hired cheap off-shore companies 12 time zones away whose developers were curiously unavailable by phone, email or carrier pigeon.
One of the great features of Drupal is its ability to run any number of sites from one base installation, a feature generally referred to as multisites .