I always have so many books on the go sometimes it can be months before I pick up a book I'm in the middle of reading again! Right now I have a little pile on my bedside table of 'Club Dead' from the Sookie Stackhouse series, 'A Glossary of Literary Terms' which I dip into now and again, 'The Children's Book' by A.S. Byatt which so far has been rather boring so I haven't got past chapter 4, 'Emma' by Jane Austen and 'Wuthering Heights' by Emily Bronte.
That doesn't include the books that I have had to move back onto the shelf so as to reclaim some space! I don't find it difficult or confusing to read more than one book at once; I rarely confuse characters or plots as they are all very distinct in my mind. Though I do have to do a bit of re-reading here and there if I leave a book a bit too long!
I often read 3 or 4 novels in between starting and finishing another one, but they do all get finished in the end!