I already knew a lot of things on OpenERP but I enjoyed a lot reading this book as it gives another point of view by comparing different aspects between OpenERP and SAP. This book as been written for people that know SAP, in order to help them discovering OpenERP. Both software answer the same need (enterprise management) but with two very different approaches.
SAP users will discover OpenERP as most concepts are described ir relationship to similar concepts on OpenERP. But it's also very instructive for OpenERP experts, like me, as it allows to understand what SAP is doing differently than OpenERP.
The only thing I regret is that it's quite short (134 pages) and, as it's interesting, you read it very quickly.