You dont really need a guide for this one contrary to what you may think, this is the first game where i got all collectables with no guide. Once you find the riddler maps which are easy to come across youre almost guaranteed to get all collectables, it just takes time. The riddles are not that hard overall, just some of the secret question marks painted on the walls which you have to line up then scan can be a bit frustrating, note that you can only see them in detective mode. Your best oppurtinity to clean them all up i believe is just before you 'go to the party' so take that opportunity before finishing the game, but you can come back afterwards if you want. Be aware, there are many trophies and riddles you cannot get until you have completed 99% of the game already and have the abilities to pull down out of reach walls etc, so dont worry too much about them until the end.
If you really want a guide to them all look here
http://www.gamesradar.com/f/batman-arkham-asylum-riddler-gui...However its much more satisfying to do it all yourself.