There is really no set methodology to cover all landslide sites except that investigating the ground profile by drilling and/or testpitting,laboratory testing of soils, groundwater monitoring, and instrumentation are common to many such site investigations.
Perhaps the foremost aspect or precursor to an investigation scheme is the observation of the landslide site and surrounding topography,along with an understanding of the local geology.
Some slides are solved without resorting to instrumentation, but invariably ground investigation by drilling or testpitting is required.
The book you mentioned would assist you in planning a scheme if you have evaluated the nature of the slide by observation.
One must have some "hunch" of cause of the failure before planning a programme. And we all do whether correct or incorrect. It is akin to a detective engaged in solving a crime. You may not always have the correct approach at the start but as more information becomes available you may have to change your strategy or realize that you have captured enough information to arrest your suspect.
There is a satisfaction that goes with arriving at a correct, but do not forget that just as the case of the detective we can also be misled.
For the experienced detective, he may be able to go in immediately with a scheme which will give the desired results at minimal costs. In geotechnical practice this takes time and is enshrined in experience and judgement.
Sometimes we get more confused with more information and in today's world we have the tools to collect a copious amount of data. The interpretation of the data, however, is generally the problem. On the other hand too litle data is equally problematic. Experience plays a role here and books tend to give some guidance.
From a personal view point, my approach is to walk the ground thoroughly, look outside of the "tunnel", formulate my strategies based on several hunches, and lastly decide on the field program. A good site reconnaisance provides about 90 % of most answers. Of course, this also depends on the size and extent of the slide. The worst thing to do is to stick around the slide material. Take a walk as the answer often lies outside of the slide area.
The above may not have answeere your question directly but provides some philosophy in relation to landslide investigations which may indirectly allow you to formulate your strategies for Bulgaria.
Regards
