If you are referring to Scott Moulton's 5-day (?!) Data Recovery Expert course, then you will be surprised to know that it devotes only two pages to electronics, and these pages are written by someone who knows almost nothing about the subject. For example, in those two pages he wrongly identifies two TVS diodes (one is a 5V diode and the other is 12V, but he mixes them up), and he incorrectly refers to them as TVS resistors. Elsewhere he refers to an IC and speculates (correctly) that it may be involved in motor control.
That said, most problems with hard drives involve heads and media. PCB problems are rare (mostly due to user error).
Hard drives also suffer from "firmware" problems, although not in the usual sense. Some of the firmware is stored in "ROM" on the PCB, but most is contained in a hidden, reserved area on the platters (System Area, SA). This firmware consists of numerous modules, some of which are especially vulnerable to corruption, eg SMART, defect lists. This is because these modules are frequently updated. If a head becomes "weak" or unreliable, then any data that it writes to the SA will be prone to errors. Subsequently when the drive powers up, it will be unable to read these damaged modules, with the result that it will not come ready, ie the user area will be inaccessible.
In short, 99% of data recovery professionals know almost nothing about electronics. PCB problems are solved, either by snipping shorted TVS diodes, or by replacing the PCB and transferring the ROM. That's something that any junior technician can do.
As for books on the subject, I'm not aware of any. Data recovery is a very secretive business.
You might like to read these articles:
New to Data Recovery ? Where to Start ?
http://www.hddoracle.com/viewtopic.php?f=56&t=176
The hard drive -- a computer-within-a-computer:
http://www.hddoracle.com/viewtopic.php?f=56&t=2600
newbie info, from and for newbies
About firmware, SA, etc:
https://forum.hddguru.com/viewtopic.php?t=6562
Tutorial - Linear and Switchmode Regulators used in HDDs:
http://www.hddoracle.com/viewtopic.php?f=59&t=231
HDDs from the inside articles:
https://hddscan.blogspot.com/2010/11/hdd-from-inside.html
http://www.hddoracle.com/viewtopic.php?f=18&t=16
Datasheets for SSD and Flash Drive ICs:
http://www.hddoracle.com/viewtopic.php?f=100&t=1577
As for a clean room ...
DIY Clean Chamber - update:
http://www.hddoracle.com/viewtopic.php?f=78&t=1003
http://web.archive.org/web/20150521062919/http://pcbusinessguide.com/forums/index.php?topic=85.0