I don't have the students write letters, but I do ask them to try out various responses with the books they read independently or during our guided reading time. I keep a crate of blank response pages near our mini-lesson area so students can choose how they are responding to their books. I only ask that they use 2 different responses each week. Sometimes I print out a key page from a flipchart (ActivBoard) or I'll make a copy of my modeled response for them to paste in their journals as a reference. My kiddos know to bring their notebooks to guided reading and reading conferences. I use their responses (after much modeling and practice) to assess their understanding of a particular reading skill or concept. They make frequent appearances during parent conferences and RTI meetings. There is so much we can learn about our students through these notebooks. I cannot encourage you enough to incorporate these into your room if you aren't doing so already.
One of the things I do at the beginning of the year is to reserve the first 5 pages for our "Table of Contents." I teach the students to number their pages and record their entries in the Table of Contents. This makes it easier (for both of us) to find specific entries for assessment and conferring purposes.
I'm including my Table of Contents today. When I have access to my school computer, I'll upload my first set of responses as well.
Table of Contents - Reading Journal