The books are in PDF format and I use iGig Book. The guy who created it is sort of a jerk and earlier versions were more of a pain to deal with because while the indexes were there often your version of the fake book didn't perfectly align with the included index. This meant you needed to insert and/or delete pages in the PDF to get the index to line up. I spent about 6 hours doing this with the digital fake books I have so they aligned a bit better making the indexing and search function work easier. I haven't updated the app in years but my understanding is that he now includes a PDF editing tool making this process far easier to manage.What format are all of your fake books in? What app do you use to manage them on a gig?
It takes a bit of effort up front but once you get it working it's really nice. Someone calls a tune, you type the first few letters into the search function and the app finds it. If none of the fake books you have uploaded include the tune, the app still finds it but it is grayed-out so you know what book it would be in if you had it.
Most of the guys I play with still just use a standard PDF reader but if someone calls a tune that they don't regularly play and have book marked a great deal of hunting through multiple books and tedious scrolling ensues.