Infectious Disease Epidemiology - A modern systems approach
The draft is fairly readable and complete enough that I use it when I teach a course on that topic. It is however not thoroughly fact-checked, error-corrected, properly referenced, etc. While I have been considering the idea of turning this into a full/real book, in my current thinking the cost-benefit analysis doesn't pan out. I thus plan to leave it online for free as is, with occasional updates and fixes, but without an attempt to make it polished and complete enough for a printed book.