Depends what level of assistance you are after.
I have just taken delivery of "Plant Maintenance with SAP" by Karl Liebstuckel - SAP Press. ISBN 978-1-59229-372-8. This book is based on SAP ERP 6.0 Enhancement Pack 4. I would consider myself in the expert user category and my initial impression is that this will be a very good reference book for me and gives me an insight into some of the functionality that I am aware of but my company has not yet implemented. It is not a book aimed at the end user and without my background I believe I would struggle to follow this book.
I have also read "SAP R/3 Plant Maintenance" by Britta Stengl and Reinhard Ematinger. This is an excellent book to give a good flavour to the reader about the capabilities of SAP PM. Much of the content is still relevant to ERP 6.0 version but there are many new features which were not available in R/3. The wording in this book is often taken straight from SAP help documentation but with the addition of some screen shots to help explain the concepts better.
SAP On-line help documentation in many areas is very good but in others it is lacking. Easiest way to access this is to press <F1> key on a field in the transaction you are interested. This will take you to an explanation of the field. If you require more information then follow the link from this screen to the Application Help.