<p>I'm looking for material on persistent data structures that can be used to implement a relational model.</p>

<p>Persistence in the meaning of immutable data structures.</p>

<p>Anyone know of some good resources, books, papers and such?</p>

<p>(I already have the book <a href="http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf" rel="noreferrer">Purely Functional Data Structures</a>, which is a good example of what I'm looking for.)</p>

It is straightforward to modify the ubiquitous
B-tree
to be persistent. Simply always alloctate a new node whenever a node is modified, and return the new node to the recursive caller, who will insert it at that level by allocating a new node, etc. Ultimate the new root node is returned. No more than O(log N) nodes are allocated per operation.
This is the technique used in functional languages to implement, e.g, 2-3 trees.