This article about behaviour and diet of Short-toed Eagles in the Forestry of Smilde in Western Drenthe /map/ is available on Natuurtijdschriften:
• Rob G. BIJLSMA, 2019 – Gedrag en voedsel van Slangenarenden Circaetus gallicus in een potentieel broedgebied in Nederland [Behaviour and diet of Short-toed Snake Eagles in a potential breeding area in The Netherlands] // De Takkeling, Vol. 27, No. 3: pp. 210-226. – 
(Nl).
The Natuurtijdschriften site was already mentioned on HoverOverUs. Here we present to your attention some more articles published in different years and dedicated particularly to Short-toed Eagles:
• Bas VAN GENNIP & Bert DE BRUIN, 1996 – Nieuw voor Groningen: Slangenarend [New for Groningen: Short-toed Snake Eagle] // De Grauwe Gors, Vol. 23, No. 4: pp. 113-114. – (Nl).
• Arno IZAAKS, 2007 – Ruim vijf weken een Slangenarend Circaetus gallicus in de duinen van Meyendel (En) [More than five weeks an ST Eagle resided in the dunes of Meyendel] // De Takkeling, Vol. 15, No. 2: pp. 114-116. – (Nl).
• Chris VAN DEURSEN, 2010 – Slangenarend waarnemingen (Circaetus gallicus) in de regio [Observations of Short-toed Snake Eagle in the region] // Fitis, Vol. 46, No. 3: pp. 125-126. – (Nl).
• Willem VAN MANEN & Rob G. BIJLSMA, 2017 – Slangenarenden Circaetus gallicus in Nederland: voorschot op een broedgeval (En) [Short-toed Snake Eagle in the Netherlands: progress toward breeding attempt] // De Takkeling, Vol. 25, No. 3: pp. 224-241. – (Nl).
All documents with mentions of STE can be found using the site search.
The excellent online database featherbase.info contains high resolution photos of bird feathers, various statistical data on their dimensions, useful comparing tools which allow to identify species with high confidence. Also, you can add pictures of your own finds there. Specific information on Short-toed Eagle is available on the separate page.
Although, this paper is rather old, it can be considered as a good example of a report on Short-toed Eagle rehabilitation. If all similar cases were described in such a detailed way, our knowledge on sexing and ageing of Short-toed Eagles would be much deeper:
• Theo van de MORTEL, 2006 – Voor het eerst een Slangenarend Circaetus gallicus geringd in Nederland [Male Short-toed Eagle found emaciated in The Netherlands in May 2006] // De Takkeling, 14 (3): pp. 232-235. – (Nl).
The article is available on the Natuurtijdschriften site now.
The following extended descriptions of observations of Short-toed Eagles in Flanders /map/ (the nothern portion of Belgium) are available on the Natuurpunt website (Nl):
• Wouter FAVEYTS, 2010 – 2010: een superjaar voor Slangenarend in Vlaanderen [2010 – a bumper year for Short-toed Eagle in Flanders] // Natuur.oriolus, 76(4): pp. 132-134. – . The article contains English and French summaries.
…and other texts:
• Dominique VERBELEN, 2014 – Slangenarend Jacobus zorgt voor spektakel in de Kalkense Meersen [Short-toed Eagle makes a show in Kalkense Meersen] // www.natuurpunt.be.
• Han LAGRING, Gerald DRIESSENS, 2015 – Slangenarend op bedevaart in Groot Schietveld [Short-toed Eagle on a pilgrimage in Groot Schietveld] // www.natuurpunt.be.
We try to not miss an appearance of papers about Short-toed Eagle on the Web. Every freely accessible article is added to the collection, a kind of specialized library. Certainly, the majority of them aren’t new, but they are not of less interest due to this. The following ones can be viewed on websites of the journal of ornithology Tichodroma and of Sovon.nl, Dutch Centre for Field Ornithology:
• Štefan Danko, Jozef Mihók & Štefan Pčola, 2007 – Hniezdenie a výskyty hadiara krátkoprstého (Circaetus gallicus) na východnom Slovensku [Breeding and occurrence of the Short-toed Eagle in Eastern Slovakia] // Tichodroma 19: pp. 75–86. – (Sk) /map/.
• Arjan Boele, Erik van Winden & Rik Winters, 2013 – Slangenarend en Noordse Nachtegaal nieuw in het BSP: van dwaalgast tot broedvogel? [Short-toed Eagle and Thrush Nightingale in the BSP: from accidental to breeding?] // Sovon-Nieuws, jaargang 26, nr 1: pp. 5-6. – (Nl).
Waarneming.nl is a popular Netherlandish database of observations on plants and animals in the nature. Such form of the data keeping becomes more and more useful according to accumulation of the data with the increased pithiness of observations (Nl). All records are connected with the on-line map; some of them contain photos and comments.