This is how I started to get familiar with SAP HCM.

1. Read an introductory book,
Grundkurs SAP ERP (Verlag , Amazon)
This book focused more on explaning the general ERP and all related industrial processes idee. Not very useful for somebody in a job already.

2. Read an Payroll book,
Personalabrechnung mit SAP (Verlag, Amazon)
Seems to be a quite good and detailed book, but hard to understand without possibility to check in a real system.

3. To test I downloaded the Trial Version
This was quite interesting, but I doscovered that it contains only SAP Basis, no HCM related stuff.

4. Read an ABAP book,
ABAP Objects (Verlag, Amazon)
Excellent book, but does not cover all aspects.
Together with the trial version I wrote my first program
This is how to import the Book Examples

5. Einmaleins der Entgeltabrechnung 2009 ( Verlag, Amazon) )
Also very good book to get functional details.

6. SAP-Schnittstellenprogrammierung ( Verlag, Amazon)

7. Now I found a great blog ABAP Trial Version for Newbies

8. This one is a must:
Personalwirtschaft mit SAP ERP HCM ( Verlag, Amazon)

9. Sicherheit und Berechtigungen in SAP-Systemen ( Verlag, Amazon)

10. Another new Blog Basic ABAP OO eLearnings

OO Programming with ABAP Objects: Interfaces

11. Another book? SAP NetWeaver Process Integration