I don't really know of any training except for some hibuck professional stuff.
The best book I know would be the DM in print (Diagnostic Manual). I'm not sure MB provides this anymore in print as it is available on their WIS system and the pay by day tech site
www.startekinfo.com . I could teach you a six month course from the info available on that site. I'm not sure but you probably could download the DM engines Volumes 1-4 and have it all.
Also available would be the theory of operation section in the WIS section, wiring diagrams, vacuum diagrams, etc., etc..
The DM tells a lot of things but most importantly its tells the enabling criteria for the fault codes you will read. This means it tells you what conditions make the action and from there the testing that is physically needed: scope testing a circuit, verifying an action, whatever.
In the long run you will need ultimate understanding of the workings and strategy used by the system to understand the diagnostic strategy used to put it right.
PS: The DM originally was sold in print in loose left updatable binders that have to be assembled from their parts if still available. There are 15-20 of them, about five feet on a shelf. They are about systems, not models, but are divided into each models application of each system. Such as in the case of LH it woul include the 104 motor and the 120 motor also and only have footnotes as to how the system might be different in a 129, 140, 124 body.
I haven't seen them on ebay, but I haven't been looking. If you could get DM- Engines Vol 2 and 3 you would be covered for LH, the whole 4th Vol is ME. The last models I saw in print were 1997. There is more coverage than that on WIS and Startek.