Hello and welcome.
I think that there is not much info about Valerian, primarily, because he was captured. Romans liked winners and having your Emperor taken prisoner must have been extremely embarrassing for many Romans. Secondly, Valerian initiated a persecution against Christians. With Constantine, the Christians came out on top and history is written by the winners; and later writers, especially Church historians, would not have been inclined to write much about Valerian.
There are a few primary sources that write about him, but not much. In De Caesaribus, Aurelius Victor only has a few sentences, ending with "For when his father was engaged in an indecisive and prolonged war in Mesopotamia, he was captured through the treachery of the Persian king, whose name was Sapor, cruelly mutilated and died in the sixth year of his reign while still a robust old man."
Eutropius in Brevarium says that "Valerian, while waging war in Mesopotamia, was overcome by Sapor, the king of the Persians, was subsequently captured, and grew old in ignominious servitude among the Persians."
Zosimus, as a pagan, in New History seems more sympathetic to Valerian and writes "He was popular and anxious to be a sound administrator." He also says that plague struck Valerian's troops, carrying off the majority. After this he wanted to make terms with Sapor and set out with a small retinue and was then taken prisoner.
Wikipedia has several other primary sources, but in all, there is not more than a few pages in total written about Valerian. That is not uncommon, as many Emperors have even less.
There are some articles on Valerian on JSTOR, mainly about his persecution of Christians, but the first one re-examines the claim of Lactantius that Sapor dyed the skin of Valerian red before placing it on display in a temple.
Lactantius, Valerian, and Halophilic Bacteria
David Woods
Mnemosyne
Fourth Series, Vol. 61, Fasc. 3 (2008), pp. 479-481
VALERIAN'S "CONSECRATIO"
Charles Oman
The Numismatic Chronicle and Journal of the Royal Numismatic Society
Fifth Series, Vol. 12, No. 45 (1932), pp. 58-62
Imperial Religious Policy and Valerian's Persecution of the Church, A.D. 257-260
Christopher J. Haas
Church History
Vol. 52, No. 2 (Jun., 1983), pp. 133-144
Two Edicts of the Emperor Valerian
Paul Keresztes
Vigiliae Christianae
Vol. 29, No. 2 (Jun., 1975), pp. 81-95
The Persecution of Valerian and the Peace of Gallienus
MICHAEL M. SAGE
Wiener Studien
Vol. 96 (1983), pp. 137-159
Why Did Decius and Valerian Proscribe Christianity?
George Thomas Oborn
Church History
Vol. 2, No. 2 (Jun., 1933), pp. 67-77
Roman Emperors in the Sassanian Reliefs
B. C. Macdermot
The Journal of Roman Studies
Vol. 44 (1954), pp. 76-80
And a coin of Valerian I
Valerian I
A.D. 256- 257
AR Antoninianus
20x21mm 3.5gm
IMP C P LIC VALERIANVS P F AVG; radiate, draped and cuirassed bust right.
APOLLINI CONSERVA; Apollo stg. left holding laurel branch and lyre on rock.
RIC V Rome 72
