Have you thought about creating your own on those subjects?? It's very easy to do, just use a software such as 'Instant Article Wizard'. It will search the web for information on any subject you type in and gather the information and put it into any type of format that you designate including your forward, index, etc...in just a matter of minutes..Then all you have to do is transfer it into some sort of 'reader' format,,such as 'adobe reader', which is probably the most popular so that your customers can open the file and read the material..Also so that you can easily deliver it to you customers without having to worry about spam filters stopping your info...
By creating your own stuff you won't have to worry about buying a bunch of books that are outdated....