I have habitually kept a wish-list of public domain books to narrate as I think of them or discover them. (There's over 90 on the list at the present!) I normally make an attempt at ranking my wish list - at least for the top one to three books - so that I can forecast my activities ahead for a few months. I keep a spreadsheet of my readings, with a planning page that plumbs in my expected time commitment each day in order to fix a target for each book's completion. Both of these I update upon completion so I can look back and see how well my planning process fared against Real Life.
That process has gotten more complicated by my shift to professional recordings. Since for the most part those narrations are the result of successful auditions and a negotiated contract for production, they must take priority over my LibriVox work. In a recent case, my audition was submitted a full year before the publisher awarded the book to me, and then they gave me 5 weeks to produce it.
So now when I slot time for a LibriVox recording, I go to the wish list and feel for which title is calling me the loudest. It's frequently not the one I had nominally placed at the top of the list.
In mid-June I had a pause between projects and I wanted to do something fun, with character voice work. "The Adventures of Pinocchio" wasn't even on my list, but when I ran across it somewhere on the Web, it was an "ah-ha" moment - I knew I wanted to do it, and soon. I started it the day after finishing "The Enchanted Island" for Audiobooks by Mike Vendetti, had one week clear, and then had to share time with my next pro project, "A User's Guide to the Universe" (which is now in final approval); still, I finished it in three weeks.
I started my formal book planning process in 2008. I finished ten books that year, 11 in 2009, 15 in 2010, 16 last year, and this year I've done 9 through July.