This Halacha is an excerpt from our Sefer
From the 17th of Tammuz until the 9th of Av one is to beware not to walk alone during the three weeks, between the 4th and 9th hours of the day. [There is difference of opinion if this refers to the beginning or end of these hours. Some write it is from the end of the 4th hour until the beginning of the 9th [for a total of 4 hours]. Others write it is from the beginning of the 4th hour until the end of the 9th [for a total of 6 hours]. Others write it refers to end of the 4th hour until the end of the 9th hour. This prohibition is relevant only when walking alone in a deserted area and not when walking alone in a settled area that contains people passing by.]
Walking in the shade:
From the 17th of Tammuz until the 9th of Av one is to beware not to walk in the border area between the shade and the sunlight, between the 4th and 9th hours of the day. Likewise, one is to beware from staying in the shade during the months of Tammuz-Av.
 Michaber 551:18; Hagahos Maimanis; Rokeiach 309; Midrash Tehillim and Eicha Rabasi 1:29; Tanchuma Naso 23; Ateres Zekeinim 549;
 Pesachim 111b [brought in Kaf Hachaim 551:225 and Biur Halacha 551:18] states that from the 1st of Tammuz until 16th the Katev Meriri is certainly found, while from the 17th and onwards it’s a Safek.
 The reason: As during this time, the demon called Ketev Meriri is found. [ibid] See Devarim 32:24; Brachos 5a; Tanchuma ibid that it is a demon like creature; The Ketev Meriri is the name of a creature which is filled with eyes and Kelipos and hairs. He has one eye by his chest. If one even looks at him, he dies. [Eicha Rabasi ibid; Shulchan Gavoa 551:53; Kaf Hachaim 551:229
 Eicha Rabasi; 1st option in M”A 551:45
 Naso Rabasi; 2nd opinion in M”A 551:45; brought ion Levush 551; Chayeh Adam133; Siddur Yaavetz
 Elya Raba 551:44; M”B 551:102
 Zohar Vayishlach p. 169; Yifei Laleiv 5:6; Aruch Hashulchan 551:39; Eishel Avraham 551; Kaf Hachaim 551:226
 Elya Raba 551:43-44; M”B 551:102; Eicha Rabasi ibid; See Kaf Hachaim 551:227-228
 The reason: As during this time, the demon called Ketev Meriri is found in the border of the sun and shade. [ibid]
 Shevet Hamussar 16; Kaf Hachaim 551:227