ADE remembers the last position in every book you have ever read.
To switch between books you need to go back to Library view and choose the other book.
If you sort by last read (dropdown at top right) it is reasonably easy.
Might be even easier still if the 'Last Read' at top left included 3 or so books
(but considering all the other failings of ADE2.0 this is a fairly minor detail).