I have both books and like both, I'd get the O'Reilly book Practical PostgreSQL first and follow it with the developer's handbook afterward. Practical PostgreSQL is available online for free as well though I really preferred having the paper copy for this instance. Along the way though, I'd really suggest you also get a copy of Joe Celko's SQL for Smarties. Time and again, it is one of the most recommended books on the PostgreSQL mailing lists.

To add to what martymart said, you should also check out http://techdocs.postgresql.org/#convertfrom, concerning 'Converting from other Databases to PostgreSQL'.