I’ve found the IBM Redbooks page, in which, there’s many interesting books like this

TCP/IP Tutorial and Technical Overview

In this one, you can find info about the different network layers, from physic to application, with  explanations about routing protocols (OSPF, EIGRP, BGP…), IPv6, LDAP, VoIP, Wifi stuff, etc

Linux performance and tuning guidelines

In this, you’ll finde some very useful info. about how Linux works managing filesystems, I/O, memory, etc and how we can monitoring this with some well-know tools like Wireshark, vmstat, top, strace and many other commands.

Also in the IBM Redbooks page, there’re thousands of books about different topics, like WebSphere, DB2, AIX, Tivoli, Linux server integration and many other IBM stuff for their servers.

PD: I’ll add others redbooks . Send me suggestions about other books