Resources for studying the Psalms of Solomon are notoriously scarce. The book has traditionally held an unsettled place in the history of the Canon. A few manuscripts of the LXX include it, however, it was not received into the Patriarchal text, nor has it found a place in the NRSV's "ecumenical canon."
Unless you want to shell out a hefty sum for a Brill monograph, there isn't much to work with if you're unable to spend hours at a well stocked theology library.
As such, I was happy to find this dissertation online.
A minor caveat: This was published in 1996. Scholarship advances. This said, the scholarship on the Psalms of Solomon hasn't advanced too much since the publication date. The book is horribly neglected by both academics and theologians and there is little indication this trend will change any time soon.
If your Greek is up to the task, the Biblia Graeca provides ready access to the Greek text. Otherwise, the New English Translation of the Septuagint provides a reliable English edition. Truth be told, the translation of the Psalms of Solomon is one of the major selling points of the NETS - it's one of the few texts that doesn't remind the reader of a slightly reworked NRSV.