The tutorials on this site are excellent. The
First Steps [
sidefx.com] series from Ari is probably the best place to start.
Peter Quint [
vimeo.com] (on vimeo) also has a whole lot of tutorial videos worth watching, some however are for older version of Houdini.
As for books, the two that exist are slightly out of date, but still fairly useful.
On the spot [amazon.co.uk]
Magic of Houdini [amazon.co.uk]
(Amazon UK links)
3DBuzz has some
free introduction to Houdini videos [
3dbuzz.com], which may be useful to get into the right mindset for working with Houdini and to learn the basics. The videos are for Houdini 9 but most of it will still be relevant.