The topx (Topics files) are books that can be accessed through the Editor which is found in the e-Sword window, top row of Icons E for Editor, Select Topic Notes, bottom of the Topic notes window there is a pop up window where you can select the topx file you want Towards the top of the window (last line of icons) is another pop-up window where you can select the chapter of the Topic Note you selected.
The next icon (looks sort of like a mountain top with a sun) is the Graphics viewer used mainly for maps that can be downloaded.
The next icon (looks like 3 books standing on a shelf that is where you will find the REFX files (Reference Books) books. There is only one line of icons drop down/pop up window is there select book, on the left side of the screen is generally a window where you can scroll to find the chapters/divisions of the book you are reading.
There is also a STEP Reader available under the Tools (right between Dictionary and Options) though not many people use it these days.
This is a quick run down.
For more details open e-Sword, hit the Help -> Contents button (or F1 key), scroll down to chapters 8-11 and Chapter 13 (specifically for Reference Book reader)
Hope this helps.