Click 'Post Reply', click 'Manage Attachments', click 'Choose file' from the pop-up window,using the new window find the file you are looking for & click 'Choose', click 'Upload'. Once your file has finished uploading you will see it under 'Current Attachments' in the pop-up window. Close the pop-up window or choose an additional file to upload.
Now you will see your attachment(s) listed above the 'Manage Attachments' button on the main window. Finish typing your post and submit.
File size is VERY limited for uploads on this site, so most of your e-books will probably be too big. If you email me the file, I can post it on my domain ( **broken link removed** ) and then I can give you the URL to add as a link in your post. PM me for my email address bro.
This file is an example, it was the only one small enough for me to post.